I got it done, and it was too easy -- and fast.
In my library district, every library has a 3D printer. All you need to do is go on-line to their 3D printing web page, and fill out a short form. You don't even have to have a library card or live in the county. You attach the 3D computer file -- that I got from your link -- and tell them which library you want to pick it up at. That's it.
I sent them the file, and an hour later they told me the lever cover was ready to go at the library closest to me -- no charge. Mine is a Martha Stewart GREEN, but a little black epoxy paint will make it look new -- and actually give it some protection.
My tax dollars at work.
I'll add that anyone who does this should add a bit of epoxy glue to make sure it stays in place. The two tiny pins in the cover are made of pretty weak, corn-based plastic, but some glue should help secure it.