Another option would be to try using a thin sheet of styrene plastic. Cut a paper template to fit, then cut the plastic. Heat very very lightly over a candle to soften it just enough to bend to shape and press into place.
I haven't used this technique for wagon covers, but I have made medieval pavilions and ships' sails this way. If the plastic heats too much and curls up at the edges, you'll need to try again with another piece!
Here's a DBA camp I made with the crossbow merchant offering a One Day Sale: