I use a 5 gallon bucket of watter, f you can find a long 5 gallon bucket it works great. Reduce the load of powder so the bullet wont plow through the bottom of the bucket. I place the bucket on the ground, and use a step to get me over the top of the bucket so I can shoot straight down in to the watter, and not have my feet below the impact area.
I mostly use this for HP testing with a full charge load, but it should work fine for your needs.