The underside got a quick coat of Osmo to seal it. It's important all surfaces are sealed to stop the wood absorbing moisture from the air unevenly and causing the top to warp. It's also important to allow the wood to move if it needs to, this means securing it to the base but allowing it to expand and contract with the seasons.
The way to allow expansion is to elongate the mounting holes, I did this to all 8 holes in an orientation that would allow expansion across the grain.
All done. Looks great, glad it's done. On to something else.