Hi Arthuy
The lamp unit could simply be glued in place where it overlaps the flange around the wing lamp aperture, not ideal but invisible, secure and serviceable. And won’t cause any difficulty changing bulbs, lenses etc..
You could possibly chop the bits you need out of otherwise scrap steel wings or…
If you want to make your own plates to mount your lights – here are my suggestions
Obtain a gasket set for the lamp, it’s the one behind the lamp you especially need as a template (usual suppliers or try e-bay as there is a regular advert for these available in complete sets .)
Take a piece of stiff card and transfer the outline of the main holes from the gasket & cut them out , punch holes in line with the mounting screws and mark the gasket outline – but leave a good margin around the outside.
“Fit” this in place in the wing and ensure the mounting holes line up (use the fixing screws & masking tape) and from inside of the wing press the card with your fingers against the wing inner surface to obtain the size required for the finished plate.
Your choice as to what to make the plate with – the main problem will be obtaining a reliable thread for the lamp unit fixing screws. Say 20 gauge steel or stiff ally plate (old boot lid, old door panel, side of steel oil can etc– cut this to your overall outline using the card template – drill / cut/punch out the holes to clear the back of the lamp reflectors – as rough as you like only you will see them.
For reliable mountings for the lamp unit fixing screws, I suggest investing in the plastic inserts as used in the steel wings, I believe J R Wadhams do these . Problem is they are square to fit a square hole – so drill the smallest hole that they can be pressed into, then fix in place using a smear of araldite or similar around the outer edge (don’t fill the threaded hole) you do need to prevent them turning. AND they need to be in the correct place or you will crack the lamp castings as you tighten the screws up.
Finally assemble all in place , screw up the fixing screws, tape the lamp housing in position and fix the plate into the wing using strips of GRP mat and resin or a body filler with chopped mat in it. (the sort sold for filling holes rather than dents).
Leave it all to set and hopefully it will do the job.
Whatever material you used slap some rustproofing paint on it or it will rapidly vanish in this notorious rust spot.
