zip, you might want to specify units on those temps

I'd be pretty impressed if you hit 35 degrees F in your trunk, at 90-100 degrees outdoors (and if your trunk is anything like mine, its probably closer to the temperature of the sun)
edit: damn you, fixing that before I finished my reply
as for the main topic... personally my trunk already has junk in it (tools, etc), so a sloppy (or "easily reconfigurable") install doesn't seem so out of place. and personally I don't open my trunk very often but it's kind of hard to miss seeing your dash.
with that said, as soon as I move on from the epia in a ghetto-rigged plexi case, to a new system with athlon mobile and a real case, I am going to make a little carpeted shelf in my trunk to slip it under... partly to make it less ugly, but mostly to keep it from getting smashed when i have heavy stuff in my trunk, and to keep the wires from getting ripped out... then again, I'm more of a 'function over form' kind of guy I guess, since my mp3 box is the only non performance-oriented thing I've spent a dime on for my car...