There's a reason most metal caps are zinc.
Bronze and brass have lead, copper is a pain in the neck... It should be noted some people also react to zinc, you can also react to aluminum if it's not oxidized/anodized though it's usually if ingested, so beware handing these then eating finger foods (and yes, you can wear through the oxidation and anodizing). Generally you want to use food safe materials. If you do aluminum you probably want 6061, it's one of the cheapest but also one of the easiest to work with and takes anodizing well. Titanium would be nice but EXPENSIVE, same with silver. Stainless Steel is actually something to consider, it's easy to machine, not too pricey and is food safe.
Something else to note, and relates to Hvontres mentioned, the stems are a hassle. The first metal caps had massive problems with this, pretty sure most of the ones sold today are from the same factory that did the first ones since they figured out the sizing, though with that bad bunch of box switches it may not even be possible to make a metal cap that fits everything. Yes, you can usually bend them to clamp bit then they may be too tight, I have 2 from some of the first ones and it's a hassle to get them to stay in place and still go onto the stem as a result they often go flying.
Regarding the small corners and such,
Machining will not be cheap, they need multiple bits and multiple sides, it all adds cost. Casting eliminates this and also offers nicer edges (other than the parting line) but its also trickier and not easy to find someone capable of doing it these days. If you just want one, I'd highly consider just getting it 3d printed or asking local hacker/maker spaces if they ever do any casting (also talk to jewelry shops they can do it). Machine shops will not be happy getting an order for just one of these, prototype or not it's a lot of work for a single, tiny part.