Get some plywood and face it with an electrical enclosure back plate and stick some Duct Seal on it.
I ordered a .22 bullet trap (branded: Highwild) and it arrived incomplete, the back plate was missing. I contacted the seller and a replacement trap is on the way. So what to do with the incomplete one they don't want returned? I'm thinking of fashioning some sort of ballastic curtain backer of sorts and using it as well.I am currently looking at 1680D ballistic nylon fabric and or Kevlar cloth on Ebay. Any suggestions would be appreciated.