will have to buy a full sheet of the plastic. I did the exact same thing to a tank of mine years ago and uses an end piece that the shop had left over. It worked just as you suspected it would and I never had any problems. I was on city water at the time that was chlorinated so I never got any critters growing. The only down side that I can see is that if you used a piece that was too thin it could crack under the weight of someone sitting down too hard. That would depend on the size of the tanks surface area. I used 1/4 inch but it was a rather small tank.