Glassing something in, can be done. I would not call it easy. Mikes idea makes more sense.
To glass it in, you need to grind/sand away any surface paint
or gelcoat--probably no gelcoat inside. Wipe with solvent, let dry and then using wet fiberglass tape glass it in. I use West Systems Epoxy. To make glue I mix the epoxy with 403 filler--so if you want to glue in a premade shelf that woukd be a start. I use 410 filler to make fillets. And of the lighter fillers from 405 which is silicon dioxide on up can be used to make a smooth radius. Then you can applied the fiberglass tape over that.
So you get the idea. It is a long job, an hour a day, and a day to dry between each small step.
Far easier to attach a wire shelf.
A fiberglass shelf wouod strenghten the shell some, and if done well would enhance the look of your Burro.