Odd question about sand domes

This is unofficial but I just thought I would through it out there. At Knott's on our 2 foot Crown 4-4-0 in Camp Snoopy we have an odd problem with our sand dome. We finally got it to work so we can apply sand to the rails, the problem is that condensation forms on the underside of the top and drips down on the sand. In short time this make large chunks of wet sand that will not move or that are already in the lines.

Anybody ever experience this problem on larger loco's? Never an issue with the c-19s.
My thought is that the temp inside the dome is not high enough and that we need a vent.(Its more or less sealed with a rubber gasket on the lid.)