This is a Hail Mary if I ever saw one.
About ten years ago I picked up a P220 and a .22LR conversion kit. The factory mags don’t have any form of last round slide hold open at all.
There was a gent - possibly Canadian? - who was making rapid prototyped magazines that did lock the slide back using the slide stop - not an interference type where the follower sticks up high enough to jam the slide open. They weren’t cheap, and I only bought two.
I’d sure like a few more. Does anyone remember these, or have seen anything similar? He made them for the P220 and P226 (I think) rimfire conversions.