With the foam in place and leaving just one side of the pontoon laminated in view of its oversized thickness, run a filet of silicone around the edges. I've used black here because that is the colour of the spray-paint we'll be using. You could of course use a glue to fix these components, and I use silicone only because (a) it's cheaper and (b) it allows fast-builds to be pulled apart and modified as part of a fast-prototyping effort.
Now to build the second pontoon, simply rinse and repeat...
