I don't know of any model of the Horsa. You might want to check out RZM for books on the glider and then try your hand at scratch building. I sure they have a book or two with layout drawings.
If I had a hankering for a Horsa for a diorama, I would use either balsa or plastic for the internal ribs, stringers and bulkheads and then cover it in model aircraft tissue and dope. You could have a field day with damage since you would build it like the orginal it would damage like the original.
Best of luck