The problem is actually in the way you applied it, in my opinion... Monokote is great stuff, but it does take some practice to be able to apply it well. Anyway, basically the mistake you made was "shrinking" it down to get it to fit... You actually want to "stretch" it out to get it to fit... Once the edges are sealed you should never have to heat the middle to get it to shrink to fit... Monokote will shrink with heat, but the problem is, it also stretches with heat. But in general it wants to return to it's "original" size as it fluxes through temp changes... What you need to do when you apply it is to tack one corner down, then warm the entire sheet a bit with a heat gun (unless your working in a very warm workshop to begin with), and then stretch it as much as you can to the opposite corner and tack it there... Then heat a bit again and stretch to another corner, then repeat and get the last one... Then keep it slightly warm as you work on stretching and sealing at various points around the edges until it's almost completly sealed... Then warm it up pretty good with a hole for the air to escape.. Then seal the last part... After that, let the whole thing cool and it will be tight as a drum and stay that way... Basically you "stretched" the monokote out, so that once it cools it shrinks a bit and pulls itself tight... Through temperature changes, it will try to return to it's "original" size which is smaller than what it is in it's tight stage... therefore it will always stay tight...