The product you used has a bonding agent on both sides. The shiny stuff peels off and you can iron it onto a backing. Unless this is what you want, I think you should have used an iron on stabilizer. It has iron on adhesive on only one side. The other side is something that either doesn't stretch at all or a knit (tricot) fabric that you would apply to your tshirt at right angles to the knit on the shirt.
If you want to salvage your block, get the lightest weight muslin you can find, peel off the shiny stuff (it's paper) and iron your block onto the muslin.