I was thinking some type of silicone, but silicone may be too soft?
FDA silicone works well. The softness is variable so I wouldn't get softer than 60 Duro - about mid range. I found the best way to cut it was with a hole or plug cutter bit. Takes a bit of practice and it's hard to find a bit size which matches stock tubes. Probably easier to cast to exact size. I've never found standard size FDA silicone washers.