Below a list of some good DIY Kayak building resources:
A former NASA engineer who now designs small boats which can be taken apart for easy storage and transport. There are also some plans for e.g. a foldable kayak seat, foldable mast, and for tape and glue joining wooden panels.
Sawfish, the smaller Clownfish, and the boxy Seafoam
The inspiration for my Nomadulino designs using a foam core and bedsheet with PVA glue covering and paint finishing.
Poor man’s fiberglass or Ghetto Fiberglass
The technique for covering a foam core with cotton or convas with PVA glue and then finishing with paint. Nylon can in theory also be used, but most glues do not stick to nylon and also do not impregnate the fiber much, so results are less predictable.
Instead of cheap and usually not transparent latex paint, you can of course also use outdoor acryl paint or a polyurethane based paint to finalize waterproofing. Or add a think layer of fiberglass epoxy for final waterproofing. What is important is to avoid long term water contact with the PVA glue layer as even D3 PVA glues are not 100% waterproof, will gradually absorb some water and become softer (and dry out again later if allowed to), and that not all other material bond well with PVA glue. So for fiberglassing you do want the top cotton layer to not be completely covered in PVA to still have enough texture / roughness for the epoxy to grab in to.
A resource for lots of ideas and tips for DIY boating stuff, and for a lot of free plans for various sizes of boats, including kayaks and canoes
A good resource for all sorts of DIY ideas, including boats, trolleys for wheeling your boat to the water, and lots of other stuff.