If you have a kayak done and ned to fit a bulkhead in front - You can cut a cardboard one out and screw on a (broomstick) handle into the center of it. Place a droplight in the front hatch. Push the bulkhead in and with the light behind, you can easily see the areas that need trimming. With a few trimmings like this you'll have one that fits in as far as you need with a pretty good fit.

Trace it on plywood, cut it out and attach the handle. Reach in the the bulkhead area from the front hatch and sand the area. Precoat some plywood to make the wood bulkhead. Cut out the bulkhead and coat the open edges good (installation time) and push it in hard so it gets trapped in there. Unscrew the screw holding the handle and apply a fillet from the hatch side with a finger / latex glove. Then fillet the front using a bent plastic knive attached to a stick.

If you have a few gaps, don't worry, they will fill with the fillets.

You can fill the screw hole from the hatch side. The rear bulkhead is similar but easier since it's reachable.