I just patched my 3D deer target with Great Foam

[slickbricky]

It doesn't work as well as the factory replacement center, but it keeps the arrows from passing through. And it only cost $2.39. Let it cure overnight or have a gallon of acetone handy.

[tlingitndn]

I shot a can of foam before. It exploded really nice. Insulation was a pain to clean up though.