On a quilt I recently finished I did string blocks. Some with muslin and other using a very thin pellum paper that I pulled off of each block. The water soluble embroidery stabilizer has several draw backs IMHO. #1 It falls apart easily with movement and since your block will not be in a frame that could be a problem. #2. The high cost of the water soluble stabilizer could quickly become more costly than your fabric scraps. #3 Water soluble stabilizer is also pretty flimsy. It will not be strong enough to support your work on the blocks. Let us know what you decide.