If push comes to shove and you still don’t have an answer to your question, I would drill down through one of the teak and grp locker lids within the cockpit area using a small drill, and drilling through one of the black mastic lines between adjacent planks. This would enable you to use something like a tyre tread thickness gauge to measure the overall thickness. Then with some careful manipulation of the probe on the gauge and and visual inspection through the underside of the hole in the locker lid you should be able to get a pretty good idea of the teak/plywood thickness, and subtracting one from the other will provide the grp thickness. Refilling the hole should then be a simple task with some white gelcoat on the grp side and black mastic on the upper side. Using a small drill and drilling from the top should ensure that you only take out material from the mastic line, and not from the wood surrounding.
