Your amp will work just fine without this. Leo and his successors knew what they were doing when they designed these amps without forced cooling. And of course we're not trying to cool the valves (tubes) - they need to be hot to work. But all the other works are inside a metal box which is heated from below by the valves, and (rule of thumb) mean time between failures is halved for every 10 deg C rise in temperature. So why not try to blow the heat away from under the chassis?

I was lying awake one night when, for no reason, I thought of the unusual footswitch design for this range of amps, which is fed with -6.2VDC from the amp's logic circuit in order to power up the LEDs in the footswitch. Why not use that to drive a fan? If you're not using the 'lead' effect, and many PRII users don't, then you don't need to dismantle the amp to get that -6.2V; you just put a 1/4" TRS (stereo) jack plug in the 'red' footswitch socket and connect to the tip.

This DC supply can deliver more than enough current to power a small DC fan. Any 12V fan will probably be fine, and because it's only getting 6V it will run very quietly. When I first tried this, I discovered the fan put some interference onto the DC supply, which came out of the speaker as amplified noise. With thanks to pdf64 on the Fender Discussion Page for his advice, I found that adding a 220uF capacitor (220 microFarad) across the fan supply killed the noise. Probably any similar value will do the same. Your amp/fan combination might not need a cap at all. 

Advantages...
Cheap some folks will already have all the parts they need in their box of bits.
Automatic It goes on and off with the amp.
Simple Experienced electronic guys can see where this is going and won't even need to read any further. You need a stereo 1/4" jack plug, preferably right-angled; a 12VDC fan; an electrolytic capacitor, about 220uF, 10V or higher, and a few screws to mount the fan.
Non-invasive Depending on where you mount the fan, you might take the upper wooden panel off the back of the amp for a few minutes, but you don't need to go inside the chassis.
Reversible Again depending on how you mount the fan, you might screw some small woodscrews into the inside of the cabinet, but you won't drill into the chassis. And if the fan fails or goes noisy, pull the plug out of the footswitch socket and carry on. Fix it some other time.

Disadvantage
Completely denies you the use of the 'lead' effect, with or without the footswitch. Due to the alien nature of what's connected to the red pedal socket, pulling the volume knob for 'lead' will add a load of noise and the 'lead' volume knob won't work. Does no damage, just sounds terrible.

Don't Get This Wrong
The supply on the 'tip' terminal of the jack socket is MINUS 6.2VDC, so your fan's positive wire and your capacitor's positive end MUST connect to the body or sleeve on the jack plug - that's the chassis or ground. The negatives connect to the 'tip' terminal. Get this wrong, and there will be a small but messy explosion.

This (below) is what I used to check the idea works. Yes, that's all there is to it. This particular fan is actually too big to fit anywhere in the amp. It draws about 60mA at 6V.

How and where you mount the fan in the amp is up to you. In my opinion the best idea is to move the air heated by the power valves out of airspace under the chassis. However I couldn't see how to mount the fan in order to do that - not enough room between the transformers and the speaker-  so I mounted my second, smaller fan (60mm) on the inside of the rear panel so it blows cool air into the amp, past the power valves. 

Here it is, mounted on the inside of the upper rear panel, just before I turned the panel over and screwed it back in position. Once in place the fan makes a strong stream of air which can be felt all the way past the 6V6s and the transformers. 

In a PRII there is a gap of 44mm between the rear panel and the 6V6 valves. To line up the fan with the gap between the 6V6s I had to centre the fan 140 mm from the end of the panel.
These two dimensions are probably different for the other amp models.

Here is my mounting method - two woodscrews. Two 15mm lengths of the outer covering of some mains flex act as spacers. The fan is meant to blow towards the screw-heads.

and here it is, in position on the rear panel. The total 'height' is about 32 mm so there's 12mm between the fan and the glass of the 6V6s.

OK, you might need to go inside the amp.
The current for the fan passes through the 47 ohm bias feed resistor and will draw a little more current  than the 2 LEDs on the footswitch (which you're not using if you're fitting this fan). The factory-fitted resistor wasn't a high enough wattage and should be uprated on every PRII even if this fan idea isn't added. I don't know what the small increase in current will do to a factory-original bias feed resistor, so if you haven't uprated it, you should do that first.

Also: I've had some helpful advice on the FDP about the possible effect on power valve/tube bias. I have measured, and there's no effect on my PRII with my valves, but for other amps this might be worth checking. In theory, if the fan's current-demand  pulls the bias voltage down then your power valves will run hotter.     

Safety / Damage Disclaimer
Valve (tube) amps develop LETHAL VOLTAGES while running, and store them in charged components EVEN WHILE SWITCHED OFF AND DISCONNECTED FROM MAINS SUPPLY. These voltages are MUCH HIGHER than mains, and higher than anything you'll find inside a transistorized amp. If this scares you, good. Inside a chassis, don't use your fingers to touch anything which isn't insulated or earthed (grounded). Don't stick more than one hand in at a time, and keep the other hand well away. Use fine-nose pliers to manipulate components. Never, ever play a connected guitar while near a live open chassis. If you're not sure what you're doing, get local help. And don't stick your fingers in the fan.

I have to disclaim any responsibility for injury, damage, loss of value or loss of gig due to inoperative equipment. Most external web pages I've given as links all say roughly the same thing, and  their content is of course not my responsibility.