An AK mag, even a 20 rd type is pretty heavy.
My answer for carrying small amounts of ammo for long guns, is a cheap nylon butt cuff. I cut a slit towards the rear and force the sling swivel stud through the hole, then attatch the sling, which locks the butt cuff in place. I usually don't fill the cuff up, just enough for a reload 4-5 rds.
Works good for small ctg rifles like .44's, .357 and x39. I also use one on my .308 scout. Larger cals like '06 start to unbalance the rifle.
But....I'm sure you can find a single mag pouch for AK mags. Heck, it wouldn't take to much to sew one.