I don't remember who the manufaturer was off the top of my head, but I have TWO styles of hi-caps for my MKII.
Both were made by the same company, one has a stainless steel body, the other is a plastic body. Been a while since I played with them, but I believe the plastic holds 12 and the SS holds 14, or vice versa.
The way this was accomplished was with a magazine spring that resembles a measuring tape, all rolled up. As it unrolls, it forces the follower up. Strange idea, but they work PERFECTLY! Outside dimensions are identical to a stock mag.
I have several of each, but considering where I am, even if I were to sell the MKII these would stay with me. It would be nice of these were re-introduced for those in Free America.