CombatBrick Mortar set. Fits all LEGO minifigures.
This comes in 4 easy to assemble pieces. It can vertically go up and down to accommodate different heights and terrain. It has one mortar shell which is kinda lacking. But it fits snugly into the end of the barrel, but not inside it. So it's not 100% accurate, but for the price it's great. You can always have a couple to get more mortar shells. The only thing to note is with rough play, the pieces probably could break easy enough. So be careful with playing and storage of it.