I like the idea of a cover plate using strip magnet but the semiphores do work and I was trying to find out if there was something missing from my car. On the Rilye the semophours sit in a box with a gasket. This stops water getting in.
On a different note I saw a lovely use of magnets last weekend. A guy had fitted a small driving glad to the front of the car with a magnet just took it off when not wanted. No drilling no mes lovely and simple.
Thanks for the suggestion, I might try to make a box to fit in the piller .