Quality silicone sealant? I had an aluminum bar included, I fanatically put silicone on it, and it's been going great for years.
Absolutely airtight anyway will not be, the top layer is too thin. Particle board is a budget item, and you have to put up with it.
Regular sanitary silicone in the color of the countertop. Dab all the way in and pull on the countertop ties.