For years people have been grafting sticking mags together to make larger capacities. For the cost of these extensions, you could buy PSA metalforms, cut some roughly in half, and weld those sections onto the bottom of a full mag to create a 45-50 round mag.
If you need to make an extended spring, just use a wire butt/crimp connector to attach the ends.
The other option is to just make a simple clamp that allows you to secure two mags together at the bottoms which would allow you to connect a 32 and a 10 or a 32 and a 20 or a 2-20's or whatever you need. It's a pretty simple design that could probably be made of delrin/similar and should be far cheaper than the TF extensions.
http://www.thefirearmblog.com/blog/2014/07/16/maghack-high-cap-magazine-adapter/
There's a lot of folks around here with skills/equipment that like to tinker/make stuff. I'm surprised none of them have already done something like this. It seems like a simple spare time project that could make someone a little extra money.