Similar problem, only I am repairing one for a friend.
Her son made it, then it broke. No point fixing it (&
tarting it up) until I get a display case.
Thoughts so far:-
1)Ornament case, for a clock/stuffed animal etc. from a charity shop. No luck so far.
2)Case for plastic flowers. Have seen a few in garden centres, but they are flimsy.
3)Display case for a doll. Either expensive or very tall. Would work with something
also in the case, e.g. flying Nazgul or trees.
4) Small fish tank. Again size & shape are the problem. Some might work if I include
some ruins.
Good luck on yours. Let us know if you have any joy.