I think the best one I have seen is two plates angled together to make a narrow "v" shape with the open end pointed at the shooter. They don't actually touch at the "pointed" end but rather have a narrow gap for the bullet to pass through. Behind that is a round or octagon shaped "cylinder" that the bullets enter and bounce around in until they lose all their energy. Best part is, no sand or fiber to clean up (it all sprays when the bullets strike). It catches all the bullets in the tube, then you have a door on the end where you can empty it out. You could just good bullet traps and see what you come up with, then copy one of the pics.
Contact me for any metal polishing needs you may have, my avatar is a pic of a standard, painted fire axe that I ground, sanded polished and buffed to a mirror finish.