Notice how that's made. It has to be that way to lay flat on the conical surface. Even a decal needs to take the conical 3D shape into account. If you just measure, and make a ring to fit, it won't. (Unless your air cleaner is flat) You need to project the conical shape onto a flat surface. Someone that does sheetmetal duct work will know how it is done. Some good CAD programs can do it too.
