The 20 rounders were originally carried in the OD canvas single mag pouch for the M14, and the canvas multiple mag M14 pouch.
When the nylon ALICE-1 LBE came out, the 20s were carried in a nylon 4-mag pouch that had an inner pull-up lifting strap for the left-most mag as you face the pouch. The four-mag pouch also had web loops on the sides into which the spoon handles of grenades would be inserted, and a small snap-closed strap would hold them secure. They fastened to the standard ALICE belt with ALICE keepers, and an adjustable-length strap fastened to a D-ring on the front of the ALICE suspenders. The pouch looked similar to the later GI 3-mag nylon pouch for the 30 rd mags, only not as tall and not having the "looped" grenade "pockets" on the sides that the 3x30 pouches do.
These 4-mag ALICE pouches are nowadays tough to find, and most that I see are really worn, some with broken closures. You might try some of the surplus web sites, surplus ads in SGN, or stores in your area, or shows. Prices for these are high, too, due to scarcity. I still have two of of the 4-mag pouches that I bought with my own money when I was still in, and I use them today for my range mag 20s.
I'll post a pic tonight.
Noah