Sure you could make a base pad work, that's what I do: cut them to length, bend the tabs out and make 171.25mm mags. I get 28 without tuning, maybe I'll put one in a vise and see if I can get 29...
ETA: welding tubes is a real bitch, but I guess if you do it low enough that the follower never touches the weld it won't be so bad. I welded a big stick from two short tubes and gave up before I got it smooth enough on the inside. It's also easy to get it too hot and distort the metal.