The best trap media is #10 asphalt sand, which is really small grain gravel. It's used on some berms, and it takes less of it to stop the bullet. The problem with a 5 gallon bucket is the bullets don't always go straight, and they "may" go out the side of the bucket if not perfectly centered. It's best to have bucket walls that are tough enough to contain a stray round, if necessary.
If you do much testing, be sure to check that you don't punch a hole in the trap media from shooting the same spot over and over. You don't want to put a bullet through the back of the trap.
To prevent splash back, if you can get hold of a truck inner tube and cut it open to give you enough rubber material to cover the opening of the trap, that will prevent most splash back. It also repairs itself to a certain degree, but will have to be replaced over time. You can shift it around if the rubber material is large enough, so you're not always shooting through the same spot in the rubber. A bungee cord around the mouth of the trap will hold it in place. A piece of plywood cut to fit the inside of the trap, and resting directly on the sand, will also help with splash back, but will have to be replaced fairly regularly.
Hope this helps.
Fred