Old cement sacks that are hardened work as great bunkers. I bought some from a local lumberyard for less than a good bag as they wouldn't just give them to me. I replaced my dirt filled (and falling apart after 3 years) sand bag bunkers and made another one. Eventually, I'll just buy some new sacks of cheap cement and do the same without worrying about having to play tetris to make the hardened ones fit just right. Also have used barrels, pallets with scrap plywood or other things across some of the openings to make a more solid bunker.
"All that is gold does not glitter, Not all those who wander are lost;" My MCB Feedback