I managed to build one out of some leftover all weather hardwood ply after building our run a couple of weeks ago. Wasn't that difficult to build, even for a non-diy'er like me. Even covered the roof with roofing felt to make it non slip. I'm going to raise it off the ground tomorrow by putting some feet on it to stop it getting damp. I've put it in the run and I have been putting straw and hay in it and the bunnies are loving it so far.
Good luck with your project.