2) Cut notches for butt and stock/barrel such that the rifle ends up roughly level.
3) Foam/rubber can be wedged in the notches, under the butt and barrel, to keep the gun secure (instead of adding a vice mechanism).
4) Cut a template from paper and trace on end pieces if extra detail is desirable.
5) Use Titebond to glue end pieces of plywood, etc. for extra thickness and detail.