I’ve used bubblewrap and large squares of corrugated cardboard sheet for jobs like this. Shipping supply stores sell the sheets to industrial customers, who put them on top of 4-foot pallets to keep things on the pallets clean and away from nailheads. If you know anyone who works on a factory’s loading dock, you’re set.
I wrap the case in bubblewrap and tape it well, reinforcing the ends. Then I put that subassembly on the cardboard, parallel to the corrugations, and sort of fold the cardboard all around as squarely and as tighly as possible. I then cut off the extra circumference, tape the seam, and trim the overlapping ends into something resembling box flaps. Fold those down, and reinforce the cardboard wrapper with acres of tape, again with emphasis on the ends.
Ugly, yes. Hasn’t failed me yet, though. And best of all, it doesn’t scream “gunbox”.
