Desperate times require desperate measures. I've twice shipped triplets, one of them an ~1898 Davis Sewing Machines of Dayton 23x23x23. For these (and the Jack Taylor trike that went from NJ to the great SW, there is nothing that seems to work better than building a box of 2x2s and 1/4" plywood. Damn the torpedoes, that got respect in the Greater Trucking World. And gave great surfaces for attaching things internally, too. With a decent drill and a pile of sheet rock screws, you can box anything. :-)
Most recently, I needed to ship a '38 Paramount frameset to Waterford to replace the rattlecan with real paint. Like most mortals, I used a bike box for that, but built internal frames to "hang" the frame. This also seemed to work well. I built bottomless boxes, just four pieces of 1/2" by 5" or so plywood (or MSB or whatever) that have the same inside dimensions as the height and width of the bike box. One goes at one end of the box, and the other at the other end. One mounts to the rear drop-out spreader maybe 10" - 12" up from the bottom, and the other to the fork drop outs (fork reversed). If you remove the fork, then a dowel (old broom handle) through the head can be capped with a couple of more pieces of scrap wood, making an "H" that attaches to the front ply. box skeleton. Be paranoid and do another for the BB if you want.
Just out of anality or whatever, I last trim the box down to minimum length required, tape it, nd ship it. Probably takes an hour or so, but no frame tube is anywhere near the box.
Finally, I took it to the LBS to ship for me. They're on good terms with the shipping fellows, and the extra outbound may help their rates. Happy to pay for that.