: Sponson are an integral, internal component of many folding boats. In most
: cases, they're the only built-in floatation and also serve to tension the
: skin. That's quite different from add-on air bags for hardshell boats.

***

Well, I just agree on this. Though, if you've read that thread, perhaps you've found that most of its substance is relevant on s-o-f's and hardshells alike.
On Nautiraids or the rawer skin yaks we can build here, sponsons are not structural.
Those posters make some points such as: sponsons don't float the yak until they displace water; sponsons hardly make additional inner floatation superfluous.
It seems, in general, that whatever innovative techniques we could introduce to kayaking, their usefulness seems to decrease as our skills increase, which has us get back to basic Inuit traditional gear, one big limitation of course being that we haven't been taught kayaking as kids as a people whose survival and food depended on sea hunt, and some side items like waterproof hull skins in lieu of game skin which had to be greased anew for each hunting trip.
I'm not even sure plywood stations are such a bargain, since structure flexibility is an advantage on a choppy sea. They had round hulls as well as hard chines. And aluminium won't float !
Inuits could have made sponsons easy if they had wanted, as they still make paddle floats. Same for floatations, by the way.
The other big limitation is, most of us paddle for leisure.
You know about this, of course, just clearing my mind.
And ranting.
You're right to find those manufacturers' sponsons outrageously expensive.
They could rather be cut from p.v.c. sheet and iron welded around a bicycle valve; or after some raw surgery, an inflatable cheap mattress could provide material and built-in valve, keeping only one tube.
Or, pool blue noodles or foam could be hung from the sides and fastened by D rings or even a large surface of velcro. In calm weather conditions they could be kept inside light p.v.c. knee tubes underneath deck.
Basically, inner volume and floatation would be the way for us who can't yet roll as Inuits, provided we don't cling to primary stability as totemic value?