I did think of doing that but wasn't sure if they liked to be completely hidden from view and being on its side would leave them in full view. At least on its side it would have no trouble fitting! would it be better off that way?
I would try it both ways and see what he likes Mine likes very very small confined spaces, like he's dug a hole in the sand underneath a large piece of slate - when I move it and made it into a cave, he moved underneath the bogwood next door You could always try angle it in an awkward shape, see what kind of contours you can get from it.