I've got this exact same problem. I've been fiddling with sollutions for 3 years. I haven't found anything yet though. I've got a BMW that's well insulated (for sound reasons) that I kind of killed because I removed part of the truck carpeting to house the computer, thus exposing bare metal that's directly heated by the sun (and is 4 inches from the computer)
My next step is to re insulate that area near the computer to keep the direct thermal radiation from the hot metal from warming the computer components. At least I'll make it heat up an insulation shield first. That should take out a good amount of the thermal energy.