A few years ago I needed to mount a couple of helm seats on my flybridge. I didn't want to drill through the cabin top because I didn't want the fasteners to show inside the boat and holes invite water leaks evenually, even with the best sealing.
I ended up making two plates out of mahogany, about 12" X 12" or 30cm X 30cm and about 15mm thick. Pretty easy to make with a table saw, jig saw and a router. I then used stainless steel "T-Nuts" to secure the seats to the plates.
The pedestals are 23cm in diameter. Locate the pedestals on the plates, locate the holes for the fastening bolts and mark the plates with large cross hairs. Then over drill the holes and fill the over drilled holes with epoxy ( I use packing tape to keep the resin in the hole until it cures).
Once the epoxy has cured drill a proper sized hole for your t-nuts and install the nuts from the bottom of the plate. This technique produces an "epoxy grommet", water can never get to the wooden plate. Run a machine screw or bolt from the top down through the pedestal and into the plate/t-nut and you're done. I use a little threadlocker just to insure the bolt doesn't loosen from vibration.
You can finish the plate with three coats of resin, glue it to the deck with thickened epoxy, then fair it in, paint it to match and you're done. Put a litle packing tape over the bottom of the plate to keep epoxy out of the t-nut threads. This technique works well if your deck is curved for water drainage. You can make a pad out of thickened resin and vary the thickness of the resin base depending on the curve of the deck or roof.
This takes a little longer than just bolting the pedestals down but when you're finished you have a permanent installation that is extremely strong and will never leak.
