I live in a small hamlet with some fairly close neighbors and occasionally I like to try out my .22 LR indoors. My neighbors don't really mind, but I don't like to rub it in. I just turn my shop radio up on some heavy metal (stuff I'd never listen to)and do some shooting at this easily prepared bullet stop. Disclaimer here: Works for me but may not for you..consult engineer.
Sloping backstop 1/4" steel, side panels about 1/8" steel for splash. Target surface, 5/8" aspenite..holes drilled along front edges ofside panels allow shingle nails to hold aspenite panel in place. Set this into pre-built to fit box, then fill box to top with sand.