Oh man, as a lover of odd guns (and accidental expert on Star pistols) I have encountered a lot of mag modifications. Metal double column pistol mags in 9 or 40 are pretty much all the same size, and present the round at the same angle, so it's pretty easy to cut a new notch to get it to lock right. Careful measuring and slow cutting is the name of the game.
Slide lock can be easy or hard. Usually, it's just a matter of figuring out what the actuator is, and blocking it up to interact properly. Thin plastic sheet as a top to avoid wear, and epoxy or model putty to build up and secure it.
S&W 59xx mags are a good host, fit into a lot of other guns, and are pretty cheap as they were a very common gun. Biggest dimensional change most of these have is not overall size, but radius; the front can be less and more square. This can bind up the magwell or other stuff like the drawbar so is worth looking at as well.