Well - the trunk is not working out so well at the moment. While I tested the PC in the trunk, I never did so during the day in warm weather. After getting everything buttoned up, I took the car PC out on its maiden voyage on Saturday in 80+ degree weather only to have it crash.
The bios reported a CPU temp of 211F - which I'm assuming is about 30 or so degrees above the normal operating temp for the VIA C7. On the bench, the temp would range from 130 to 180F and I never had a crash.
So, I need to consider:
1) Better ventilation - I'm thinking of running some flex hose connected to air scoops that I would discreetly attach somewhere. Also run additional fans.
2) Relocate computer to the cabin (behind the seats on the shelf) or build a double din unit and mount in the dash like MPR90 2002 Mazda Miata CarPC