You can use a round O-Ring 3/32 Cross section ( Thickness ) X 1 3/16 I.D. (Series B70123) or Metric 2.5mm C.S. X 30mm I.D Material should be Buna-Nitrile which is the most common O-Ring material. For the two mounting screws Parker Hydraulics has a 5mm (4.83mm I.D. X 9.78mm O.D.) seal washer. they are about $8 for 5 piece bag. If you are in the U.S. McMaster Carr has these P.N. 93786A100 and P.N 9452K87 for 100 O-rings.