Hi Karl,
I sell offcuts of UHMW on ebay and in major woodworking chains like Lee Valley and Woodcraft.
You can't actually glue UH to anything if you want a durable, strong joint, because UHmw is a closed molecule that won't even absorb water, let alone the glue components you're trying to create a bond with.
The best you'll ever be able to achieve is an "atmosphere bond" with self-adhesive tape. As has already been noted, if you don't have a lot of shear strength needed, that may do all right for you.
You CAN weld UHMW, but it takes special equipment and a specialized skill. Probably not worth it for a single job.
I think your original idea of using rivets is probably the best bet for the job.
The idea of crazing the surface by etching it or using a propane torch is iffy. UHMW melts almost at the same temperature that it ignites at
- you could have a real mess from that.
BTW, if the UH DOES melt, it smells like candles, but the drippings are NOT wax. They'll cause 2nd and 3rd-degree burns. Be careful.
Eric