Step 1
Measure the length, width and height of your seat (for the height, subtract the thickness of the decking, which will be attached to the plywood lining later). Take these measurements to Bunnings. My seat is 440mm high, 400mm wide and 2000mm long and my decking is 20mm thick so I've subtracted 40mm (20mm for each side of the seat) from these measurements. Use Bunnings cutting service and ask them to cut 12mm H3 treated pine plywood into the appropriate sizes. This will make transport, handling and construction much easier.
Step 2
Cut plywood to the appropriate lengths and assemble the lining. I used masking tape to hold the panels in place while I glued and screwed them together.
Step 3
Paint the outside of the liner with Bituproof plus and the inside with acrylic paint.
Step 4
Insert a couple of bracing timbers to hold the liner walls in place and paint.
Step 5
Using a drop saw (you can hire one from Bunnings if you don't have one), mitre cut lengths of decking timber to go on the outside of the liner. All the corners should join together on a 45-degree angle.
Step 6
Using the mitre saw, cut out the timbers for the seat top/lid. This should be a frame made from decking with lengths of decking inserted into it to form a solid surface.
Once you are happy with this formation glue together using a two pot epoxy glue. I used Ados Rapid.
Step 7
Glue and screw a couple of bracing timbers to the underside of the seat at right angles to the inserted decking described in the previous step.
Step 8
Move the seat into place and attach to the joists of your deck using coach screws.
Step 9
Attach hinges to the lid and secure to the box. I've got my lid opening toward the deck rather than away from the deck because the seat is sitting hard up against a slat screen making opening away difficult.