Have you found anything to use for the top mounting plate for the adapter?
I have a small lathe so it was pretty simple to use a small piece of thin aluminum, machine it into a circle and drill. HD, Lowes both sell aluminum strips. Easy to cut with a hacksaw and file to shape, though not sure if they sell it in the right thickness. The thickness of the plastic lens is about .040 inch but there's a little wiggle room there. If you don't have a lathe you could use the washer, put a bolt
through it, tighten down with a nut then chuck it up into a drill and hold it against a piece of sand paper, abrasive stone, etc.. Might be faster/easier than filing.