The problem with the switch is that it was a bit too long to fit in the corner. I had to bend one of the mounting lips to a 90° angle. The mounting holes of the switches are very small and since I didn't have small enough bolts, I had to drill the holes out. Of course, the head of the bolt I used was too big, so I had to grind it down a bit etc etc ... Finally I only used one bolt and some silicone to keep it in place.

Finding the right fan is indeed not a simple thing to do. The specs of the fan I used are 0.6W, 5500RPM, 4.9CFM, 0.12in H2O, 26dBA.
Sunon also makes 6mm fans (but they're 5V only so you'd need to modify the circuit) and 8mm fans (but they're more noisy at 32.5dBA).
I wouldn't use anything thicker than 10mm as it is a thight fit as it is. The capacitors on the back of the display couldn't be in a more worse place . I don't think that even a 8mm fan would be a direct fit. You'd still have to tilt the display a bit.

To lower the voltage at the fan, lower the value of R2. My initial plan was to use a 1K trimmer pot, but used a fixed resistor to keep it simple. The problem with a lower value for R2 is that it lowers the entire curve and doesn't make it steeper (on the contrary). If you want to make it steeper, you'd have to get a thermistor with a higher 'B' value (a measure of the thermistors sensitivity to changes in temperature, higher B values giving greater % changes in resistance). Downside is that the temperature will probably stabilize at a higher value too.
I have no idea of the B value of the thermistor I used. The place I got it from didn't even know about B values. They only labeled their thermistors by resistance.
I actually bought 3 thermistors, 220, 470 and 1K2 and they all gave very similar results (steepness of the curve). In the end I used the 470 because it was easier to install.
So you see, finding the right thermistor isn't easy either.

The fan turns off when the voltage goes under 5V. If you want to keep it inaudible, you'd need to keep the voltage at about 5-6V because the holes in the top lid create a lot of turbulence. The fan made a lot less noise when not attached to the lid.
If you would do that with the thermistor I used, you'd need to use a 400-500 ohm resistance for R2 (thanks for the graph Genixia), but than you'd never get 12V at the fan when you need it.

I have the player sitting at my desk while I'm typing this. With the fan turned off, it stabilized at 38°C. With the fan turned on, at 28°C. This is a very good result, but it isn't really necessary. The empeg is perfectly happy at 38°C so there's no need to have the fan go on. I might look for a thermistor with a higher B value, add a trimmer and do some more testing. In the mean time, I disable the fan when I take it in the house. Iit usually stays in the car so it's not really that big a deal for me.

I'll add this info to my installation page.

Frank