You can still builkd them. If another 1994 "crime bill" ban gets passed you'll be out of luck.
I think the best way to go about building a hi cap for the hi point would be to braze a tower to a Suomi drum. You'd need to add a chain of 12 or 13 dummy rounds to push the ammo through the tower though.
I've often wondered if it would be possible to modify the high point to accept a different mag such as the S&W or glock. I think there's enough room in the grip for it. I've said a hundred times that if the hipoint could accept another standard hi cap mag I'd own several.
They really missed the mark there when they designed it.