I made a couple of houses with large roof spaces ontop to act as extra levels, here is the link to a thread on one of the ones I made
My lollystick house & roof terrace :)
Poor Willow was in a 5 level Imac fantasy at the time so I made this to fit the contours of that cage. Although she has moved to a bigger better cage now she took this along with her when she moved and some 8 or 9 months later she still sleeps in the house part everyday and has her litter tray and sand bath on the top terrace part, she loves it and it really was well worth making
I did a couple of them for Ebony but she chose to eat hers ROFL
Good luck with yours