I worked on building a test rig this winter also. Haven't used it but a couple times because of weather. I used a 50 gal poly barrel.
I put a piece of tractor tire inner tube over the 8 1/2 x 11 hole and hold it in place with 1x1's with silicone and screws, fill it with water and hang stuff from the top slot. I shot it twice with a 300WBY and 185 Bergers, it was to cold outside and the first shot split the tube a little but the second shot sealed back up. I may need to back it with neoprene to keep it from splitting but it seems like it will allow for multiple shots without resetting. The Bergers blew up! They lay in the bottom in little lead droplets and copper slivers. It lay in the bottom starting at 4in and ending after about 14 in.
My goal is to shoot actual hunting loads at full velocity at extended ranges, making it repeatable allows for shooting a string of shots then driving down and checking results.