The Czechs know how to make cheap but durable surplus. 

Czech desert camo is a neat pattern and to have it on your butt is even better. 
These butt packs be worn easily on an ALICE belt or anything else in a similar size. 
They open via a hidden zipper and have more straps that I know what to do with.
 
I'm sure you would need the rest of the Czech gear to properly use all of these straps, but I did not find any issue setting it up on my belt. 

The bags measure approximately 9.5" x 10" x 2". 
 
Since these are surplus, please expect fading, stains, markings, small holes, and/or fraying. Otherwise, these are all ready to be used.