I've done a few round mags, 2 PTXs and my good friend Kenzo's Delta68:
I'll try and get a closeup of one of the PTX mags tonight, my son's stuff is here at my house. It's certainly doable as far as the hole in the mag. Getting the spring to stay in well is a little tricky, I'll see if I can find a description of this before I retype the whole thing!
One thing I would REALLY recommend if you've not done this before is to practice making holes in the sides of plastic 10rd tubes. This will give you cheap, disposable, tryout pieces to get your size and shape settled before you have to touch your marker!