I want Sig to manufacture a Sig 320 FS in 45 acp and 40 sw

They did, up until recently. Model numbers 320F-45-BSS (.45 ACP) and 320F-40-BSS (.40 S&W).

And there have been several batches of FS .40 and .45 police trade-ins specifically that hit the market over the past few months. Places like Aim Surplus and SportsmansOutdoorSuperstore had big batches just a few weeks ago, but they're sold out currently.

You can also get one in a different configuration, like a Compact, and then try to source a FS slide and grip module.