While I can't speak for Burro
... most of the older, smaller rigs used a barbed-channelled automotive-style bulbed rubber gasket, similar to what you find on cars.
The barbed-channel pushed right on over the edges of the curved door ... with the bulb pushing up against the fiberglass body.
As I said, I'm not sure if this is what Burro did. One tip would be if the thickness of the door tapered down around all the edges. If it does, I'm sure you'll find something to fit (maybe with a little glue) at an automotive store. J.C. Whitney, on line, carries a bunch of sizes, etc. ... but it would be difficult to say which one would fit your situation.