Attach and reinforcment of cut shoulder forms.
Use drywall screws and secure head to shoulders after mixing 2 part foam, do this with haste.
After the foam has set and is hard, using a 1 inch spade bit and bit extension. Drill a hole long enough to place a 1 inch wood dowel into the shoulders and the neck of the head.

Simple hole, augured out.

Wood dowel cut shorter one inch than the hole is deep. Elmer's glue or polyester resin can be used to glue the dowel in place.

Once the glue or resin has dried or set. a little bit of foam will cover the holes.

Using dowels is much easier and faster than cutting a ditch for a 2x4 and foaming it in place, much quicker too. total time was 5 five minutes other than allowing the Elmers to dry.
I used four 1 inch dowels in this kudu form. at different angles. You can do more dowel if you feel the need. This has worked well for me for 30 years.