Thanks for all the additional comments!
The holes in the mounting plate are 11mm - and currently I am using 4 x 10mm bolts so I guess I could look for 7/16" bolts which are closer to 11mm although there doesn't seem to be any movement between the bolts and the mounting plate - just between the mounting plate and the wooden platform it sits on due to the holes in the platform enlarging over time. Of course looking for 7/16" bolts over here in Turkey
won't be the easiest - it's strange a French company makes the holes an odd size...
I have discovered another complication - there is very little clearance between the holes (especially the ones furthest from view) and the underside of the cockpit
floor above - certainly not enough for an electric
drill to be held vertically above. Fortunately there is space below so I'll just have to do things upside down
I could definitely bolt larger mounting plates (above and below the wooden platform) as you suggest antoha. I was initially steering away from using stainless steel in case it reacts with the mounting plate but the bolts that have been used for the last 4 years are stainless steel and haven't caused any problems so I think SS plates will be ok.
Should 4mm thick plates should be enough?
I'm keen to recess the top plate into the wooden platform 2-3mm so that I can bed
it in epoxy resin to ensure it's level as well as to reduce the additional height the plate will add. It looks like the top of the plywood
platform is already coasted in a layer of fibreglass (see the initial photos) so this should be relatively easy with the Dremel tool I hope...
I've checked the overall structure of the shelf the mounting plate is bolted through DeepFrz and everything seems ok there - no signs of movement of cracking anywhere.
Any other thoughts?