PJ, I used 2" square tubing with 1/8" wall and its plenty, in my opinion. However, building one on casters thatll be moving around will need to be constructed a lil differently. Youd definitely wanna make the feet much wider to keep it from tipping over while being moved, especially since the center of gravity will be so high. And itd probably need to be sturdier also, so maybe 3/16" WOULD be the way to go. Personally, I wouldnt wanna be moving it around on the rotisserie (although I HAVE slid mine around inside my garage, but not very far), especially since yours is a convertible. Speaking of which, you do realize that you need to brace it very well before putting it on the rotisserie, across the door openings, etc, since convertibles lack the rigidity of a coupe, and are much more prone to twisting and/or flexing. Hope this helps, and good luck with it.