Take a matching length of square-section spar and fit two of the laths that we made to either side to create a 'shoe' that we'll use to bond a ski to each pontoon.
We have to go to these lengths because insulating foam is generally of the order of 30kg/cu/m and not really able to accommodate screws, even given various types of plug used for driving them into plasterboard for instance.
Foams of around 100kg/cu/m accept fasteners, but these tend to be exotic types like PVC which are provided by specialists and cost a lot more.
I once used them extensively but the world has changed since and it is now entirely feasible to build practically 'disposable' boats in great number: like BIC biros.
Incidentally a metric cube of foam weighing thirty kilos might not sound overly light to you or I, but it is compared to one of water that weighs around a tonne.
I drive chemicals around in metric IBCs or Intermediate Bulk Containers, and you don't want one of those dropping on either foot.
