There are skin boats that have forms - just like a bulkhead except that you cut the middle out leaving an inch of wood all around the outside. Then they have stringers that travel the length of the boat and pass over these forms in mortices. In this case all you would have to do is fill in the form opening with either a fabric or solid material with silcone adhesive calk.

When I speak of the water that skin boats take on, I don't mean leakage, I'm refering to the horrible event of a wet exit. I know from years of experience that the bulkhead boat is easier for and assisted drain or a self-pump out after reentry. Whereas the skin boat with float bags holds and enormous amount of water - not only since the bags don't come up close to the cockpit but also because of leakage past them. They're hard to drain, pumpout etc. Some low volume ones won't support a paddler doing a paddle float reentry without flooding to the cockpit lip (capacity).

If I had to - on a challenge make a bulkhead for my Greenland style kayak I would use fabric and tack it in with an exectric glue gun and calk the he-- out of it with the best adhesive, flexible calk I could find filling in the gaps around ribs etc. The main problem would be the front one getting in or working through a front hatch opening of some type.