Wellnuts are the preferred method for fastening many things on your kayak. As with rivets, inside access is not needed. The wellnut is merely a brass thread insert over some rubber. A wellnut works like a shield or molly you would use to fasten items to a wall or ceiling in your home. When inserted with the flange up and then screwed, the rubber on the inside will expand and hold while making a watertight seal. Rivets might have slightly more holding power but they can leak. The wellnut's ease of use makes it the clear choice for your fastening needs.
Nuts and bolts are the strongest fastening method, but do not create a watertight seal. If you have don't have access to the inside of the kayak wellnuts are the best way to go. For most non load bearing applications (rod holders, fish-finders etc.) the wellnut is going to work fine. Picture below.
Some Tips On Using Wellnuts:
We sell wellnuts in our store.