Those remind me of the delta kites I used to modify for use on longish BT60 based flexie RGs. Had to make my own flexie 'tops', where the spars all connect, as typical kites do not have this. Also, had to use a crossbar type deal for springing open the wings and holding them out. Remember, this was a flexie *rocket glider*, not the tube-ejected BTC float away version!
If any of these things show up locally, I might grab a few and see what can be done with them. They refer to 18", but is that span or length? In either case, wouldn't be able to do beyond a BT50 with those.
Biggest issue with those kite-based flexie RGs, is trimming. Hand tosses are not very accurate, and if you 'dork' in the first few flights, you might not have enough salvageable tubing to continue
Best one I ever made was back in '93, which incidentally flew GREAT on the just released Estes E15s. None ever CATO'd in this particular version, but did in other gliders!
Literally flew that thing to death, until the main BT wore out and the kite plastic got too charred in spots. Never got another one to work that well ever again. Did upscale using this tech, and an 8 foot cloth delta kite, for 38mm H and low I motors. That did not glide as well as it should have, probably due to the heavier components.
You never know what cool 'toy stuff' items you can run across that come in great handy!