My friends all have fancy travel bags including the Chain Reaction oe. It comes with these awesome frame spacers which look like great idea. You put them in place of your wheel and pop the thru axle back in and it stops the frame or fork form getting crushed. Given that 2 of my mates have lost bikes in boxes to this failure, I wondered if anyone makes them? Essentially aluminium tubes that drop right in to the wheel space. They have them different sizes depending on your frame boost 142 135 etc.
tbh i reckon the best thing to use a nice block of a wood and drill a hole in it and use your axles. Won’t damage your frame and will be nice and wide and stable. You can also tie the rear mech to it with some string nicely tucked up into the space between the dropouts.
Or or what I did when I last sold a frame. Cut a length of M10 threaded stuffing longer than you need, fit in the dropout with 4 x M10 nuts, two on the inside against the frame, two on the outside of the dropout. Nip the nuts up.