Way back when, GI 20-rd mag pouches were available and cheap, both cotton and nylon. Not anymore, unless they're really ratty, or you get real lucky.
As an interim measure, you can carefully insert a styrofoam block of the correct dimensions into the bottom of a 30-rd GI (or other) pouch.
First is to find out the correct dimensions of the block. You want to be able to insert the block without cutting the nylon mag seperators, which help to reduce mag rattle. Shouldn't be a problem.
Second is to make sure that the pouch will still drain water, I.E, the block does not obstruct the drain hole at the bottom of the pouch. How you do it is up to you, but test a water-filled pouch w/o block to see how it drains, and then test with block. Your pouch, with block, should take LESS time to drain, as it has less volume. If your pouch doesn't have a water drain, get another pouch; All gear must be able to drain water, period.
Third is to wrap the styrofoam block in duct tape––three layers on side nearest mag lips, so styrofoam doesn't break off and clog the open end of the mag.
Easy, cheap and do-able.
For custom pouches, google FMCO, and see what they offer. ALICE Pouches can easily be adapted to MOLLE set-ups, especially with the use of wire-ties or MALICE clips from Tactical Tailor.
The styrofoam is good for floatation if you go into the drink, but the space below the 20-rd mags can be used for all sorts of survival supplies, as long as the drain hole is not blocked, and the supplies are not frequently accessed/replenished.
Official GI load-out was 2X4 20-rd mag pouches, plus as many bandoliers of ammo as you wanted to carry. Early M-16 bandos had 7 pockets, each containing 2X10 clips of ammo.