On the one I just put together I used machine screws and nyloc nuts. Riveting panels is easy there as the rivets can just be drilled. At least here they don't drop the back of the rivets inside the tube frame. You could go to Dzus fasteners I guess but the tunnel shouldn't need pulling very often.
http://www.pegasusautoracing.com/gro...oupID=HARDDZUS