The prototype arrived from SendCutSend and I like it. This material feels like a good choice - 1075 spring steel @ 0.4mm thick. The letters look nice, all the tiny cuts were made perfectly. The shim performs well with this material. It's nice in the wallet - it weighs 8.3g vs a credit card weighing 5.4g.

The steel has a blue color from tempering. In some areas it got blasted off. I may give it a new finish.

The standard attack works well. I do think it needs a softer edge to slip on the latch though. I tried deburring to smooth the edge but the deburring tool skates across, it's so hard. I'll have to sand.

Attacking the front of the latch is not effective in this stiff material. This only works on the simple double-door scenario, where you would usually have a push-bar anyway.