In the past when shipping rifles I've used 1x4 or 1x6 pine for the edges and 1/2" plywood for the sides, 3/4" styrofoam on each side inside, bag the rifle and as Nor Cal Mikie described, fill the box with spray foam. I've never had a problem other than people complaining of overkill. Often what I'm shipping is original rifles where the cost of overkill in shipping is not an issue . . .but yeah, shipping, insurance, etc will often go well over $100.
The crate I received from Kibler was (IIRC) masonite sides on a plywood frame--and broken in on one end where the end got pushed in (because screws going into voids in the edge of plywood have little strength). Because of the superb way Kibler had the parts braced and attached inside the box, there was no damage or parts loss. The next time I ship a rifle, I'm going to copy some of the blocking Kibler used, in addition to my usual overkill.