I had a Gretsch marching snare arrive some time back with a badly warped wood hoop. We had some discussion here as to a method of repair, and this is what I eventually did. The hoop is maple-ply, 14 inch,and needed to be soaked in hot water until pliable enough to be fit onto some kind of form to hopefully bring it back to true. And the form I eventually landed on is a needlepoint hoop. I found some at the local hobby store, one size was 14 inches. So, I soaked the snare hoop in very hot water in a big stainless mixing bowl, with another inverted on top to keep in the heat, long enough for the hoop to become slightly bendable, then fit it onto the needlepoint hoop. It fit so tightly, that no clamps were needed. I lined up one edge of each hoop, and set it on a flat surface to check for true, and it looked OK. I let it dry for two days and separated the two... It's now installed on the Gretsch. Now, this is the only time I have ever done this, so it may be a total stroke of luck, but in this case, the hoop was not usable in it's condition, so I had nothing to lose.