Maybe a form made of 2X8's with plywood bottom, fill with plaster paris and lay half of the 3-D target in to get imprint. Lay opposite side in 2nd form.
After form is complete, put 2 halves together tightly with starps, drill hole in top and fill with foam. Would have to grease the inside so when you remove straps the target would seperate.
Just an idea, have never tried myself but would be an interesting project.
I did make some plywood hogs one time with hole cut out for vital and covered with black felt, attached rebar behind legs to push in ground.
A good shot would blow right thru felt, bad shot would stick in or blow thru plywood.
They were fun and lasted awhile.