I've got several bakelite 20s that a member of "that other forum" made for me awhile back. Basically he cut the midsection out of a 30, JB-welded the bottom back on and shortened the spring appropriately. He did an outstanding job; the mags have held up through hundreds of rounds of normal use, as well as being left loaded for long periods.
Can't remember who the guy was, but I believe "HiCap" (of 40, 50, 60rd mag fame) will do a similar job. Heck, it may have even *been* him that did mine, before he started doing it as a side business.
All that aside, I believe there were factory 20s produced, at least in small quantities. I've seen a pic of a 5.45 Krink with a PBS-5 suppressor and a 20rd mag, I think it was in the Japanese AK book. Mag looks identical to mine except for the black stripe mine has around the base to disguise the seam.