: I'd like something a bit more permanent that I can push in and leave, without
: worrying about deflation... closed cell foam comes to mind, but I don't
: know if that will work, and it can be expensive.

Permanent seems bad if you want the insides to dry out. But if the reason
for avoiding commercial airbags is cost and availability in the right sizes,
you could try a mesh bag filled with 3 litre wine bags. The mesh bag would
be a perfecltly tailored fit for the boat, and be easy enough to pull into
place with a line. Wine bags are free if you use wine boxes for cooking (or
even drinking - they are good for paddle trips). They are not as durable as
commercial airbags, but I use them in whitewater boats where they get a lot
of hammer (repeatedly pulling them in and out, inflating and deflating to
pack round flask, lunch, gear, etc.) and they last pretty well. A single
bag failure is no big deal as that's typically under 10% of the volume in
that end of the boat and I have a couple of dozen spares at any time...

Lovely smell when you let the air out, too :-)

Andy