Hi Peter
One time at Mulfab I spotted some off cuts of thin ally sheet that Peter had from his radiator manufacturing...in exchange for putting the kettle on he gave me a handful of off cuts and I planned using them for switch brackets etc. Then I hit the same problem you have, compounded by the brass thread inserts in the laps backing onto 'fresh air' instead of a body panel!
I spent the next couple of hours making 6 'backing discs' which I spent a happy afternoon fitting between the lamp bodies and the mounting plinths..in effect giving me a backing plate. One the lamp bodies are fitted and tightened the lenses can be removed without the brass threaded slugs moving. It's a winter job mate, good radio on and a pot of coffee time ;-).
