If I want to seal something above the water line that I will be able to get apart easily in the future, I sometimes use clear RTV calk from Home Depot. I set the parts fastened together with the calk in place, but do not tighten the fasteners until after the RTV has dried. This method works best if you have a very irregular surface under the part that you need to mount. If you need to fill in a very thick area with RTV, you may need to wrap the edge of the part with masking tape to keep the uncured RTV from running out. If you want to make a gasket instead, then you need to start by checking how flat the mounting surface is. If you have a gap of 1/8" or more under a flat edge, then please consider the method above or consider using a thick foam rubber gasket. If the gap is 1/8-1/16" then 1/4" closed cell foam rubber should work. If the gap is 1/16 or less, then 3/16" 40-duro rubber (outdoor material/grade/color of your choice) should work. Harder rubbers don't conform as well. If the surface appears flat, then you should be able to use most any standard outdoor gasket material.