Build a Deck-Top Pond

From two 8-foot-long 1×8 cedar boards, use a circular saw to cut two 30-inch-long sides and three 15-inch-long pieces for the ends and divider. On the piece that will be used as a divider, measure 3/4 inch from one long edge and rip-cut the board along the marked line.

Temporarily clamp a side to the plywood bottom panel with equal overhangs on both ends (Deck Pond Project Diagram). Drill 1/8-inch countersunk pilot holes and remove the clamps. Apply waterproof polyurethane glue to the edge of the plywood floorboard and screw the side to the floorboard. Glue and screw the two ends to the floorboard and side; then attach the other side.

Paint the inside of the box with rubberized sealant, or install a pond liner (a liner is preferred if you're planning to add fish).

Fill the garden compartment with potting mix and your choice of plants.  Top with an ornamental rock, if desired.