How to make a concrete bench
Step 1.2 Cut the plywood, plastic, and assemble the bench form
Cut a rectangular piece of ⅝" (15mm) thick plywood, 40" long x 22" wide (1000mm long x 550mm wide).
Also cut a rectangular piece of ⅛" (3mm) thick *plastic sheet, 40" long x 22" wide (1000mm long x 550mm wide).
Lay the piece of plywood on a flat even surface.
Then lay the plastic sheet on top of the plywood sheet and the 'picture frame' on top of that.
Temporarily hold everything in place with clamps while you screw the all the pieces together.
Fix (glue and nail) strips of ½" (12mm) *angle fillet (f) around the inside perimeter of the bench form against the frame and the plastic sheet.