Adding a hatch and a bulkhead to a sit in kayak provides buoyancy and also storage space both when on the water or transporting. Provided the curvature of your rear deck is not too great and you have space this is a great upgrade to your yak. There are pre-made bulkheads and hatches available on the internet that might suit your exact kayak but this was what I did to reduce the cost.
Step 1: Locate a Hatch and Ensure You Have Enough Flat Deck Space.
I sourced the above hatch from Austin Kayak and they were super helpful in measuring them over the phone for me before I ordered.
Step 2: Materials and Tools.
Fasteners. Length and quantity to suit your hatch.
Bulkhead material. You can buy precut for your yak ($70) which might still require trimming and you can also buy sheets of thick foam to cut your own. I created a sandwich from 1 sheet of styrofoam for stiffness and two 2' square interlocking foam floor tiles for appearance, resilience and water sealing. Styrofoam thickness dependent on space and stiffness.
Marine goop or 3M 5200 marine sealant. (Photo shows I bought 2 but only needed one to complete the project)
Cardboard for templates.
Drill, saw or jigsaw(better), knife, scissors, file, sandpaper.
Step 3: Installing the Hatch
Tape a string between the ends of the kayak to establish a center line. Position the hatch in your chosen location. To close to the end and the deck might not be flat enough and bow the hatch too much. Too close to the cockpit and you might not have enough room for the bulkhead. My deck created a small amount of bow to the hatch which splayed the rim outwards slightly and simply created a tighter fitting hatch. Once the hatch is positioned tape it down on its flange and remove the hatch cover. Mark the profile inside the rim with a pen. Marking with the pen held vertically leaves a small rim inside the hatch to provide more material for the fasteners and the sealant. Remove the rim.
Drill a single hole inside the edge of the oval shape and then jig saw along the line as smoothly and carefully as possible. Use a file and sharp knife to clean up any imperfections. (Note - you can keep the piece you removed for any future kayak repairs.)
Position the rim in place and drill through the holes on or either side of the center line. Install and lightly tighten these screws to hold the rim in place as you drill the remaining holes. If the rim is distorted slightly by the deck profile drill some out board holes and fasten the rim down lightly before drilling the intermediate holes to ensure alignment.
Lightly rough up the deck surface around the hole with sandpaper to help the sealant adhere. I left sealing until the bulkheads were cut and ready to seal.
Step 4: Cutting the Bulkheads
Position masking tape around the inside of the kayak where the bulkhead will go. Measure both sides from an edge like the seat support to ensure it is square. This will be on the hatch side of the styrofoam.
Bend and fold cardboard to fit inside and cut if roughly to shape. It does not need to be exact but as long as it fits in and doesn't have to many big gaps. Tape this rough template in place along the masking tape line.
Cut a 2 inch disc out of cardboard. Push a pencil of pen through the middle of the disc and then roll the disc around the inside of the kayak drawing a line on the rough template. Remove the rough template and place it on top of a fresh piece of cardboard. Using dividers or a compass draw a line 1" outside the line your drew on your rough template. Cut this new template out neatly leaving a bit extra board in areas the 2" disc might have driven over like the ribs in the base of the kayak.
Install the new template in place against the masking tape line and trim any tight spots. If you have any gaps mark where they are and simply cut a new template out until you have a good shape.
Use the template to trace around and cut out the polystyrene sheet. Push in to place inside the kayak and tape in several places on both sides.
Use the template to cut out two more sheets using the floor mats. Note that due to the tapering of the hull cut one slightly bigger and one slightly smaller erring on the large side since it is always easier to remove material. Push them into place and if they belly out too much then remove a little material around the edge as necessary. A belly is ok if it pushes flat easily with your finger. Mark the pieces as to where they go and which way they face.
Full dress rehearsal. Remove everything except the line of masking tape and re-install everything as a double check to ensure they are marked correctly and fit well before caulking/sealing.
Step 5: Sealing and Final Assembly.
Rough up the inside of the hull alongside the masking tape. Apply a bead of sealant all around about the thickness of the styrofoam away from the masking tape. The syrofoam can only be pushed into position from the larger cockpit side. Position the polystyrene and push into place up to the masking tape which pushes/smears the sealant as it is pushed into place. Remove the masking tape and let cure (unless you are impatient in which case ensure you support the polystyrene from the opposite side when installing the outer foam pieces).
Run a bead around the seams on both sides and apply the odd dot or stripe across the middle also. Push the foam piece into position and repeat on the other side. Push from both sides in the middle and around the edges to ensure the sealant is squeezed into any voids.
Finally run a bead all around the outside seam of the foam on both sides. This was more easily achieved by standing the kayak on one end and then the other to get gravity assist. Access behind the cockpit lip can be tight and you might have to finger it in a bit at a time!
Once the bulkhead is completed and the beads smeared neatly, run the remaining sealant around the inside of the hatch rim and bolt it down to the hull gradually, front, rear, sides and then the screws in between pulling it down evenly. Wipe away all sealant the squeezed out the side and smear the inside neatly between the small deck flange and inside of the rim.
Step 6: Job Complete, Just Let It Set.
Attach the hatch cover, let the sealant cure and enjoy your added flotation and storage compartment.