‘lower case’ is more commonly understood by Native english speakers as ‘enclosure’
individual Hulls …segments
Sails … flanges.
The bottom of the board does not flex/stretch all that much, even with a flexy deck, so the holes for thee screws do not have to be huge, to allow even significant flex.
Also the gasket friction between deck and enclosure, will be fighting the flex.
I used wide pan head or truss head screws, no metal washers, but did add rubber washers under their heads for a little better of a seal and hide ithe imperfection of my insert installation.
Even aiming for high precision of threaded insert locations, once they were installed, i needed to open up the holes in the enclosure a bit in order to thread the screws.
Basically the bottom of the segmented part of the enclosure will open up the most when the board flexes the most.
The screw holes just need to be a smidge bigger than the screws themselves, and that smidge might be a requirement anyway as perfectly drilling and threading the inserts into the deck is not a task that allows precision, with hand tools.
Metal Washers might rattle, but spreading the load with a washer is a wise idea in my opinion.
I think instead of threaded inserts on my next build, I am going to epoxy threaded studs into the board, and have nylock nuts on flange bases, maybe a rubber and metal washer sandwich under the nut.
The threaded studs require drilling much smaller holes in deck, weakening it less, and should allow for easier alignment on assembly.
Also less worries about epoxy getting inside and corrupting insert’s internal threads, or stripping or cross threading the internal threads, or pushing screw through grip tape on topside of deck.
One could elongate the holes in enclosure a little if one expected a lot of deck flex.
