Crossmember strength

As my designs show, my plan is to connect the 2 pontoons using 2 box sections and a deck. Below is a detailed design of the box section, showing 3 bulkheads and openings for storage.

Cross member, 200 x 20 x 20 cm

My initial plan was to make the sections¬†using 18 mm plywood. That would be plenty strong I think. But it would also be plenty heavy. So I’m considering using 9 mm plywood. Or maybe use 18 mm for the verticals and 9 mm for the horizontals. But can I get away with that?

How much strength do I need?

The pontoons will weigh around 100 kg each when finished. I plan to use them for storage when underway and I will probable have one or two batteries on board. Let’s say the total weight of each pontoon could be anything up to 200 kg.

Total width of the boat, and therefor the box sections, is 2 meters.

In an extreme scenario one pontoon might be in the water and one might be suspended in mid air, for whatever reason. That means each box section need to be able to carry 200 kg / 2 (box sections) = 100 kg as an absolute minimum. To account for dynamic loads, I would like a safety margin of 100%. So each box section needs to be able to carry 200 kg at 2 meters length.

Anyone care to help me calculate / guesstimate the minimum required wall thickness?

Box section, x-ray view showing 3 bulkheads inside