[MissileStick]

So I ran the numbers. assuming a box 10" x 8" x 4", 1/4 inch Lexan would dissipate 150 watts at a 25 C temperature difference. So the thermal shutdown starts at 75, measured at [close to] the MOSFETs. Assuming it's 30 C out (86 degrees)...

MOSFETs are 167 F (75 C, when the throttling begins)

box air temperature is 131 F (55 C, or 36 F cooler than the mosfets; anyone with a case and CPU thermometer want to sanity check this?)

outside temperature is 86 F (30 C)

So even with that rather large temperature difference between the MOSFETs and the inside air, you're dissipating quite a lot of heat. Most of the heat is going to go out the heatsink, of course. If the inside temperature was higher, still more heat would be dissipated through the Lexan. If the box was bigger, same thing. Ditto if the lexan is thinner.

Are these assumptions good? Is that too high an operating temperature?

EDIT: also, whichever material you choose remember to paint it matte black! Silver or white wouldn't radiate away heat as well.