Make one out of your choice of fiberglass or vinyl panel. These trailers were last made almost 40 years ago.
Fortunately you have the frame, which is the most important part.
On my trailer, I got lucky and it still has the original panels. But the center panel is smooth, while the panels on either side are corrugated. It would be relatively easy to recreate them.
I've seen some modern units people have bought from the new Trillium
company. They look a lot different, I don't really like them. And customer service from them is spotty from what I have read.
Do a search on their customer reviews via google. Lots of disturbing stuff, here's an example:
"We have had nothing but hassles with this company. Prepaid for an awning
plus installation and taxes last Sept. to date we still have not received. Promises of a refund but that did not happen, now taking legal action. Trying to find licensing on this company but there does not seem to be any. So perhaps doing business without a license