I came across a company on the internet specialising in seals and bought a few metres of two types for the less than watertight lockers on my Bayraider. After much trial and error, I found that the following was the most effective in keeping water out:
1. The outboard (horizontal) edge of the locker should have a seal strip where the bulb sticks out horizontally from the rim. Vertically might seem to make more sense (the bulb sticking upwards at right angles to the rim) but I found that the locker would not shut.
2. The inboard edges (three of them, two vertical, one horizontal) should be lined with a seal where the bulb sticks out at right angles to the rim. For reasons that I do not understand, I found that most water was entering the lockers from the two lower corners of the rim (backwash?)
3. It is not necessary to seal the fore and aft rims unless you think you are going to sink
4. The seal company recommends that you glue the seals in place. As the seal slots have tenacious metal jaws which fit over the rim, I am going to see if I can do without. Their two-part waterproof adhesive is pretty toxic stuff, including toluene amongst other ingredients.
I have field-tested this combination with a powerful hose from every conceivable direction and not a drip has entered the lockers since installing the seals. An added benefit is that if the locker lid accidentally falls shut, instead of a loud crash, it sounds more like the closing of a VW Golf door. Photo below.