Just off the top of my head, the 9mm, .40, and .357 Sig are nominally the same size cartridge & gun if you start with something that'll fit the slightly larger .40 case....
A single-stack .45 magazine has to be at least wide enough to hold the rounds
, and the grip and a few other parts may have to be larger, too. Sounds like a whole other gun to me, even though there would be a lot of common parts. I think the 9mm being a semi-double-stack (I hadn't heard of that) is just because the slightly wider .40-ish frame allows it, and that should add one or two rounds. The .45 magazine has to be the same thickness, more or less, as a 1911.
Just me, but making a .45 in this design might be a bridge too far - IMHO, the .40 as an SD round is quite good enough without the extra size issues. But who knows.... I originally thought the .40 was a little silly until I started research (and playing with them). Just seems to be reasonable now.
Regards,