Years ago I used to make my own cork floats for tanks and once in a while for a carburetor or two.
Go to the hardware store and somewhere in amongst all of the specialty screws, bolts, nuts and stuff you should be able to find an assortment of corks of all sizes.
Grab one that is bigger than your tank float, sand it to shape and size.
Then you will need a Dow Corning product called Deracane 411-350, should still be able to buy it from any aircraft parts supplier like Wag-Aero, Aircraft spruce, etc.
Give the float a couple of coats and you should be able to use it once it has set up good for a day or two.