I've actually tried flashing inside riding mower tires. It failed almost immediately. I actually think the edge of the flashing was cutting on the tubes. I planned to cut the tread section out of OTHER mower tires and stuff it in there as a liner but never got to it other than on some small 10" diameter 4.10-4 rims on the front of some rear engine riders. THOSE tires I cut easily with a utility knife! The kind of tire you'd need to cut up to use as a 'liner' on your tire would be pretty aggravating to do the cutting on, i think. I'd buy a purpose-made tire liner in this case for that reason. Keep in mind since tire liners are basically sized for the tires outside diameter and have nothing to do with rim diameter (minus the thickness of the tread part of the tire), you could use a liner from some other tire of similar diameter even if it's labelled for a totally different wheel/tire size as long as that tire works out close to your ~43" tire height. Probably about 40" inside diameter in there since your tread bars are probably over an inch tall and the body of the tire is probably 1/3" or so. It's ok if it's a little bigger because you can just slice across it (i've heard diagonal instread of straight across) and overlap it a little and the tube will just conform around it.
If you want to try making your own, try it on radial light truck tires, not bias ply or other tractor tires. Reason being a radial is going to have less layers in the sidewall and be easier to cut there. You CAN generally drag a utility knife through car/truck tire sidewall, but it's going to be a lot harder on your wrists than the tiny ones i did up! Use the 'hook style' razor utility knife blades so you don't have to push the knife so much to keep it in. If you have to cut across the tread there's going to be steel mesh in there. Reciprocating saw will just vibrate the flexible workpiece back and forth instead of cutting it, so you're better off with a metal blade in a circular saw. It's gonna smell bad.
If you have metal wire showing at the end make sure to buzz it flat so it doesn't poke the tubes. Snips then hand file or just hold it up to a bench grinder is what i would do. I didn't have to worry about that with the tiny tires i cut up as liners because all their cords are just nylon. Heck, something even easier would be to just go get a bunch of that rubber floormat stuff they have on rolls by the foot at the home improvement stoors in the flooring section and just cut it down to width and roll out about 3-4 layers in there.
I'm throwing out a lot of educated guesses here because i haven't actually done this on a large tire YET but i DO do a lot of car/truck tire mounting/dismounting/repair as well as all my smaller tractor stuff myself. I'm an auto tech teacher so i just take stuff to work and use the machines there (not that our machines would work on your tire!). The above is what i can piece together from the large amount of automotive tire experience i have and the experiments i've done on up to 15-16" wheel tractor tires so far.
Good luck!
