Ive generally found that trollies are available always at airports, and when there havent been, surprisingly enough, the floors in terminals are usually smooth enough that Ive been able to push the box along sliding and it works ok.
so when arrive at the point where you will leave your box, will you only remove the wheels? From the outside and leave the plates on for the flight?
You did say it will make a scraping sound when dragged, so thats why I picture it without wheels on but the plates still there.
good idea on the cloth and glue for "handle holes". Ive always put bits of tape to reinforce these areas, and its generally been ok, although the handles have gotten ripped a bit a few times, but liveable.
with your bike at 21kg 46lbs, that is heavy, a good deal more than my Troll, 14-15lbs more, so that does make a difference for schlepping the box on its own if a trolly isnt around, so I can see why you came up with an easier solution.
all the best with safely packing the bikes, Im sold now on the "zip tie the front wheel into position" so that the tire goes up against the crank area and headtube area, and the zip ties keep in firmly in place so that the front hub is safely in the frame triangle dead space, and cant ever move against the frane.
Judicious placement of foam bits are key also.
touch wood
For the last bunch of trips, Ive even brought some of the key foam bits with me, as I have space in my large rackpack ontop of my panniers, and they weigh nothing. I also bring extra zipties with me also, again just handy to not have to source them and the number of grams of all this stuff is really really small.
good luck with the flights and baggage handlers (touch wood again)