Mike,
Thanks for the suggestion, but I there is a little information I had previously omitted (as I didn't think it was pertinent). That is, my plane actually has two cabin lights, one in front and one in back. That, I think, would require two timer circuits.
I don't think the fact that the relay is closed while the master is on should create a problem. The amperage draw should be very low, and I don't think that would create any appreciable wear on the points. Am I wrong in those assumptions.
Don