Know I once read that large holes in foam 3D targets could be repaired by wrapping tightly in Sarah Wrap or duct tape, poking a few holes and filling with spray foam.
Checking at the hardware store today see several varieties of spray foam, so guessing the large gap type gives the best fill, adhesion and toughness?
Before and after (for the 4th time). Repared this one yesterday with foam. I nail a piece of carboard to the back first. I then cut some pieces off old targets, put them inside the void, then fill with foam and nail cardboard to the other side. The cardboard will keep the foam from swelling and I think allows it stay more dense. Let it dry a couple days and it'll last a long time.
I really like the idea of the Nerf footballs. I'll try it the next time.
Foam rollers used in PT work quite well.Find at resale Gw,etc for around $3. Cut them to match thickness of animal..then fill in the voids with LocTite dense foam
The key to making a patch last is to have a bunch of targets and spread the shots over all of them. I patched the coyote over 10 years ago and am still shooting it occasionally. I have 21 other patched targets but seldom set them up now.
sack suggested the foam "rollers" used in Physical Therapy work. Yep, they work.
So do those "yoga blocks", also made of a denser foam.
A little bit of sculpting on either the roller or the blocks can get a basic insertable/plug shape, with the spray foam used to hold them in and smooth over the plug.
Both are relatively easy to find at yard sales, and thrift stores, if you have either in your area.
I found a trick that I use with polyurethane glue, it expands like the foam when it interacts with water. While it's drying I push it back down into the repair, 3 or 4 times. It makes the material denser and and less of a mess to cut away outside of the animal later. It works pretty good.
I have 25 3 D targets and most have been repaired. For the larger holes I force old t-shirt in them. That helps slow the compound shot arrows down. I use the clear wrap that comes on a small cardboard tube and wrap around the target as tight as I can. I then poke a small hole in the wrap and use the Great Stuff foam to fill the gaps. Wearing rubber gloves I push the foam around to fill the gaps.
I also repair a lot of targets. One thin I do is to use a tube of silicon window type sealer, and spread it out like from a tube of toothpaste all over the target. I then use a scraper to spread it out into a thin layer all over the target. When it dries it is like have a new smooth skin on the target. You can get it in various colors or you can paint it. It makes the old faded wrinkled skin look almost new. Cost is about $5.
A little more involved, but a really nice repair. I think the key is getting them filled before they are totally shot thru.
It would be nice if every club could get someone to take on this responsibility, even if there was a cost involved it would still be lots less$ than buying new.
I used old arrow shafts poked through the halves at different angles to align and stabilize the parts for wrapping and foaming. I pulled them out after the foam set up.