Step 1
Measure and cut your exterior material to form a box that fits the space you need it for. I've used 4mm marine ply.
Step 2
Glue the interior framing to the ply which will strengthen the box once it is formed.
Step 3
Glue, tape and pin the box together. The pins and the tape will be removed once the glue is set. I used five-minute epoxy for this, which is strong and quick but can set before you are ready, so use slower-curing glue if you have the time.
Step 4
Remove masking tape and pins then finish the outside of the box by planning down any uneven edges and sanding to round of the corners.
Step 5
Cut the polystyrene sheet with a sharp knife. Fix the polystyrene to the inside of the box, using PVA or any other strong water-soluble glue.
Step 6
Measure the inside of the box then cut and fold the aluminium sheeting to form a sleeve that will fit snugly inside. Seal then rivet the joins, using a waterproof sealant (check with the staff at Bunnings). Coat the back of the sleeve with Bituproof Plus and allow to dry thoroughly.
Step 7
Insert the sleeve into the box. Finish the top with a section of 90 degree aluminium angle which will tuck under the top of the timber framing. Rivet the 90 degree angle into place.
Step 8
To make the lid, cut a polystyrene insert to fit the marine ply top. Glue to the top panel with a strong water-based glue. Form a piece of aluminium sheeting to go over the polystyrene and surround with 90 degree angle. Paint and attach lid fastenings.