I got one of those dash-mount kerosene-filled aircraft compasses to put in the car; the rear seal was gone. I cut one out of a bicycle tube and it seems to leak slowly; I think that either it's not be the right kind of rubber, or there's a weeny little hole somewhere.
I know there are latex tubes and butyl tubes - this is the butyl kind, and fairly thick (for a bike tube).
If the r.c.m. advise is that the rubber's ok, I'll have another go at making one; otherwise, is there a better gasket material?