I went to wally world (and have used Home D's too) and bought one of the 18" deep, 30 gallon storage 'tubs/totes' and filled it with rubber mulch, shook it around for a couple min and refilled it after it settled, repeat until it is full and does not settle any more.
I used #6-3/8 SS screws to secure the lid more firmly, every 4 inches all the way around because the two securing clips can not take the weight of the mulch when turned sideways.
Set it up turned on it's side and staple targets to the plastic lid. Or screw on a 'bulldog' clip to the top edge hang targets from.
After shooting over 100 shots from my re-valved, 3800 PSI Texan using 364gr slugs at 900 FPS I had to screw a piece of Coroplast to the front as the original cover was too shot out and mulch was falling out the big hole. As it is shot out i have used 1/4 in paneling cut to fit the top and screwed on and other things but I get the Coroplast cheap so like n use that.
After about 4000 rounds (45, 357 and 25 cal) and multiple top changes the plastic rim was too worn out with too many screw holes so I poured the mulch into a new $5.00 tub and started over. Now if I was shooting just the smaller bore stuff it could last many thousands of shots and years but the bigbore stuff makes the trap face only last 4 or 5 months.
I have never had a 45 single slug go through even at point blank range and they only seem to penetrate about 12 to 14 inches. The mulch resettles around each bullet as it penetrates but I have had to add a couple of handfuls of mulch after 500 bigbore shots or so. I keep it in the backyard full time as it is all plastic and rubber and is not harmed by rain.
I do have a 4 x 4 triple sheet of 3/4 plywood bonded together with Liquid Nail adhesive behind the tub as a lightweight backstop just in case to keep any missed shots from going too much farther ... even the 45 slugs drop fast after going through that. Mainly I did this if any slugs were too close to the side of the mulch and deflected out the side, but I have been careful to only shoot the bigbores at targets centered on the tub face to minimize that. with the smaller calibers i am not worried about side exits so much.
Note I used to have a 24 x 36 1/4 steel plate at a 45 degree angle (giving a 24x24 target area) to deflect bullets into a sandbox which worked but the clang disturbed my neighbors. I live on the edge of a ritzy subdivision (built long after I was here, and I bought when there was nothing but miles of nothing around me, sigh) on 5 acres {400 foot long shooting lane/range} and with a good LDC and a trap like the mulch which is mostly 'silent' when the slug strikes they do not know I am shooting, haha