I think the biggest thing here when he says break down for transport is that the one listed in the original post fits in a backpack so you can ride your bike to the water wearing it on your back. You then blow up the pontoons, put it together, and attach your bike.
Could that be done as a DIY? Yes. Thing is, this looks like the type of thing that's fresh out of the gate from a crowdfunding campaign. It has lots of custom made parts, has been extremely well thought out and designed, which is why it's very expensive. You're paying for the idea, not so much the thing itself. Building that EXACT thing from scratch will take a LOT of time, effort, trial and error.
Building something LIKE it could be fairly straightforward and inexpensive, but sacrifices will need to be made, such as the ability to transport in a backpack.
Something with a rigid frame, perhaps with telescoping crossmembers to make it narrower could be pulled BEHIND the bike with relative ease, using a pair of small lightweight bike wheels off a burly trailer or kids bike. Pull a pin, pop them off, and stow them on top somewhere.
The bike could be braced vertically with a pair of struts extending up from the frame to a collar that goes around the seat post.
Back tire could sit in a pair of spool shaped rollers that then drive a rear mounted propulsion system, whatever that may be.
Front tire could sit in a channel that swivels to turn a rear mounted rudder via some brake cables or a pushrod.
That way, the weight of the bike can still sit on the wheels as intended, and your weight would provide ample friction to run the drive rollers without having to design a mechanism that clamps to the bike frame and contains a gearbox and flexible output shaft to a front mounted propulsion system.
It took 5 minutes to come up with an idea to build something LIKE this that would be reasonably inexpensive and simple, and that I could build after a few hours shopping online and a couple days in the shop. It would take me a couple weeks in a machine shop, and a wide assortment of skills (that don't come cheap) to duplicate that exact thing.
…...Joe
