I have a flexible panel I got off a friend because it flew off his van and hit the road as he was driving at 60 mph. It had 2 damaged cells, after shorting those it just worked with a slightly reduced output voltage indicating all remaining cells we OK, I was pretty impressed it survived that. The flexibility does seem to be fine, and bending once to match a curved boat roof is exactly what they're intended for. The one I have is 2-3 mm thick, aluminium backing, EVA front. I probably wouldn't bother with a gap, if you make it large enough for useful air circulation then you'll have lost all the advantage of a flexible panel. If mounting without a gap, then thermal contact between the aluminium back and the steel roof would be good as the board can then heatsink the panel a bit.