I had to get a few items at the local home improvement store. Took a look at the plywood they have. Wood panels can be cut to length (at extra cost) and they also offer various "pre-cut" panels.
They have 1200mm*400mm (47"*16") plywood panels in various thicknesses ranging from 3mm (˜1/8") to 10mm (˜7/16").
In order to make assembly as easy and fast as possible, I would probably settle for one of those pre-cut panel for the deck. A thickness of 5mm (1/5") looks decent to me.
The plywood panels I found were rated for indoor use only. Marine plywood seems a bit less common in the average home improvement store and is more expensive. I assume that once treated with wood sealer (probably G4 marine primer just like on the Sequana) hull would be OK.
Anyway back to SketchUp: here's a new iteration of the barge, this time 1200mm*400mm.
Compared to my previous design, this one is obviously 50% longer - 1200mm vs 800mm - while keeping the same beam.
Skegs still project 10mm from the hull. Instead of the bulky "front skegs" of the previous design, this one only has a streamlined taper.
Overall still a crude design. Inside is still missing structural beams.
Let's consider the inside of the hull is 110cm by 40cm and we want the barge to go down by 4cm. This amounts to 110*40*4 = 17600cc, that is to say 17.6kg (39lb).
1.5L water bottles are around 33cm high with a diameter of 9cm. Soda 2L bottle are the same height and are more "box-shaped" with a base around 9.5cm*9.5cm. Current design above has 11cm of inner "headroom" which might be on the short side once you consider the structural beams that will be added to the bottom (inside) of the hull.
With three rows of four bottles (4 bottles side by side, pointing at the bow), I should be able to get at least 12 bottles of either type in the hull. If beams are carefully placed 100mm apart bottles would actually rest on the bottom of the hull in between the beams.
12 bottles would give me 18kg (39.5lb) or 24kg (53lb) depending on bottle type. Might even be able to squeeze an extra bottle in ... Bottles would of course be filled with water from the pond to keep barge as light as possible during transport.