You will have to make a float. Check home improvement and hardware stores. You want a cork in the natural form as apposed to compressed granules. Gasoline could dissolve the binder. Another option is the Mercury closed foam float (Mercury number 1385-8983 – NAPA number 18-7208.). You will need to cut it down to fit as well.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.
