One tip is to replace the existing screws with nylon ones, if you can find any (they're readily available from Farnell, for instance). Do them up fairly tightly, but not so tight that they shear off, and they'll stay that way more or less permanently. The stretching of the nylon acts as a thread-lock device. Of course, if they DO come out, nylon is non-conductive...

pca
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Experience is what you get just after it would have helped...