I ordered this item for my diamond key that the remote options don't work on:

3 Button EWS Remote Key Control Circuit Board 433MHZ or 315MHZ for BMW Keyless

https://www.ebay.com/itm/193908942077

$8.50 delivered.

I'll replace the included CR 2032 battery with a name brand one before sealing it. That'll probably make it last longer.

Currently, it looks like two of these would be about the same price as a two-pack of replacement diamond keys, which could be scavenged for their circuit boards, but I only need one.