Watch Gaskets "O" ring seal for case back, Diam up to 40mm, thick 0.5mm to 1mm
The smaller gasket will stretch and won't "slacken out" when you close the back. Zenith 200M diver's watch - case No. 19.0130.393. 30 x 1mm. you can use size 32mm and stretch it manually to the size required then.