I started making carrier boxes probably 20 years ago for my live steamers. This is the first one I ever made for my original Accucraft 2-truck Shay. It's a self-contained unit - nothing detaches, so nothing gets misplaced.
The bottom has the typical sawn grooves for the wheels to sit in. The sides and back are padded with 1/2" Ethafoam, while the front and top are padded with 1" Ethafoam.
The back is rigidly attached to the base, while the sides, front, and top are hinged. The top is slotted to receive and hold in place the rabbits on the top surface of the sides and front.
When all folded up and the front latch set, everything is held together and the loco is compressed within the Ethafoam.
Brass corner protectors, a brass handle, a brass compression latch, and rubber feet finishes it off.
One of the first times I used it was at the old Queen Mary Show. I had my stuff stacked on a hand truck with a bungee cord vertically around things to hold them in place. The Shay, within its case, was on top, and I was wheeling everything towards the outside elevator. When I got to the elevator, I let the hand truck tip forward to rest on its base. Apparently, I wasn't gentle enough in the tipping, because the Shay came loose and fell forward around three feet, landing upside-down and at an angle on the concrete. Luckily, it landed on one of the corner protectors.
I got it back on the hand truck and up into my room. I was petrified as I opened the case. The shay received absolutely no damage... not even a scratch. Nothing was bent and nothing was broken off. The corner protector on the case was mildly dinged up.
I made several more carriers like this, each custom sized for the loco it was to hold. For example, the carrier width was established to allow 1/8" compression into the foam on each side. The same went for the back. The front usually needed some custom foam arrangement to hold the pilot and the smokebox front, again allowing for 1/8" compression into the foam. The carrier's length was adjusted accordingly to achieve the desired compression. For rod locos with tenders, I made two carriers... one for the loco and one for the tender.
The only modifications I did to later carriers compared to the first one was to install 2 latches on the front instead of one (as a safety measure), and to employ folding handles instead of rigid ones, allowing the cases to be stacked.
Hope this is helpful/inspirational.