The most effective way to stop leather from stretching is to stop using leather that stretches. This will eliminate all headaches.
Don't use the neck of a hide don't use the belly of the hide sometimes the flesh side will have blood veins showing it indicates a loose top grain in some instances.
The best hides are finished on top and back side of the hide and they are split to a specific thickness 6 to 8, 9, 10 or 12 ounce and so own. If you don't want crappy projects don't use crappy leather.