Fixing flats and switching tires today. The flap on my spare turned out to have split in the old rubber, so I took off a tire from another split rim I've found. There I only found a short piece of flap held in place by the valve stem and just covering the split by one inch. The short flap tapers off in thickness by the ends.
Getting a new one takes a week or more, so I wonder if anyone has used something like this with success? The tube where I found it holds air, so it must have worked back in the day. A possible drawback could be unbalance - all of the extra weight like the split mechanism, the valve stem and the flap is concentrated in one area. But as a spare it won't be used much, so unbalance wouldn't matter much..
I use ceramic balancing beads in the tubes of the wheels in use.
Well, I looked further in my stash, had another split rim to plunder, in there I found a good flap that goes all around
Will leave the short flap for some other day if I'm really short on flaps..