The bag you use will depend on the what the stage is asking you to do. For me, I've made my own bags based on existing designs, and used their dimensions (which are publicly available online). Out of the 6-7 my wife made, I settled on 3 that serve different purposes, but may all be used during a stage.
Tab Gear Str8 Laced - This is a rectangular rear bag with a heavy plastic bead fill (from Michaels).
Armageddon OG Gamechanger - Use this on barricades to provide a flat shooting surface to balance the rifle on. Again, used 6lbs of heavy plastic beads from Michaels)
Wiebad Pump Pillow - Used as a space filler between elbows, knees, etc... filled with stryofoam beads for a bean bag chair. super lightweight.
Check out youtube for videos of the last two to give you a better idea of how they work. If you're feeling especially "gamey"....get a tripod and a tac-table!