Finished this brooch, which was an experiment in riveting.
The perspex and brass were held together with temporary rivets while I filed and sanded the edges, then separated and cleaned up separately before assembling the brooch (see Bicycle Parts).
The corners are held together with rivets made from small silver discs with fine silver wire soldered on the back. The enamel sample is held on using ball rivets made from sterling silver wire, and the silver shapes have fine silver wire soldered on the back. I cut small notches into the brass at the back to stop those pieces from rotating.