Mooh the Mechanic! As you know, your "attaching" project is extremely difficult because of the forces involved.

Have you ever seen an ACRI thumbpick? It has a bronze thumb loop, with a thin short Delrin pick section riveted on the bottom. Rivetted like a grommet, not like an ironworkers rivet. Speaking only as a beginner, I love this pick above all others. But in any case, I mention it as a resource for you, in looking at one successful way of fastening a flatpick to a thumbloop.