I started doing this a few years ago.....
When my 3-D deer starts getting a sizeable hole in it, I get a wide (18") roll of that wrapping film that they use to wrap palletized goods.
I then wrap the deer where the hole is, and 6" to 8" past the hole on either side. Then I poke a small (about the size of a pencil) hole in the center of the film where it's over the hole in the deer target.
I then get a can of that "Great Stuff" that hardware stores sell for sealing windows, or seams, or whatever, and I stick the nozzle tube in the hole in the film, and in a circular fashion, I fill the void in the target, working from the middle out.
You have to make sure that you fill all the voids, so that when it dries it's really dense. In fact, a way to tell if you've done it right, is if the stuff oozes out form the access hole, and even on the edges of the film that you wrapped the target with. Don't worry about these, they will be cut off and rasped down after it's dry.
In a couple of days, remove as much of the wrapping film as you can. You may have to trim a bit, but for the most part the Great Stuff will conform to the deer target's contours quite adequately. I then take flat brown spray paint and color it.
I actually had my last 3-D deer target get SHOT IN TWO, and I was able (with flexible tubing for support) repair it this way and I got another 3 or months out of it before I had to do it again.
After the second time I gave that target it's final reward and bought a new one!
Offer No Apologies.....
NRA Endowment Life Member