I had been wanting a roof-top platform for 30 yrs, ever since I first saw the picture of AA ("Camera and Lens," p. 63) on top of his International Harvester. About 10 years ago I built a removable one of sand-varnished 3/4" plywood attached to a Yakima rack on top of my Volvo V70. Looked weird as hell, cut the gase mileage, and was "removable" only with the help of friends -- but it worked great. After a few years, the ply started to separate, and I thought of replacing it with marine ply, but when I found how expensive it was, decided I might as well go with planks: easier to put on/take off. I thought I'd use thick cedar boards, but was convinced to go with Ipe, since it was rock-solid and I could extend the platform farther out beyond the supports. Well, 6-foot planks worked beautifully, looked great, and I got some nice photos ... but the wood was SO heavy (it doesn't even float) that its weight broke one of the support towers ... twice.
The wood's back in the basement, and if I do this again, it will probably be on a pickup truck with a VERY heavy-duty rack system. In that case, I'd extend the wood over the cab. In the interim, the Volvo now has >200K on it, so I just climb on top of the roof....
(a displaced Minnesotan myself)