I have some 28qt tubs in my 41qt rack. I just slide the tubs far enough so the back hide is sitting over the heat, but there is still enough room for the snakes to get off the heat. Works fine for me... It's not an ideal situation IMO, but this was a rescue situation and I decided to keep them in the tubs they were used to so I could get them eating and up to weight. I just haven't changed them out to 41qt tubs yet because they're all doing so well...
EDIT: I'll snap a pic in about 30 minutes when I get home from work so you can see what it looks like with the 28qt tubs in the 41qt rack.