Initial Build

Bottom Frame. 1/2" exterior plywood, 7'x4' outside dimn. 2x4 frame border.
Side wall. 5/8" plywood 30" high.
Front wall.
Rear side wall.
and Miz securing the last side wall.
Top frame of 2x4 on top edge of side walls.
Uprights between the top and bottom frames.
Raf lining the inside with some expanded polystyrene foam sheeting, to provide underlayment before laying the pond liner.
Pond liner (Firestone 45mil EPDM 10x15') roughly draped into pond.
Slowly filling with water, and smoothing out liner into edges and corners as weight of water pulls liner into place.
Top frame (2x8) secures liner. They are centered lengthwise on the top 2x4s , so there's a 1" overhang into the pond. Excess liner trimmed off.
Added a corner ledge for the filter, with the base for the filter covered with scrap pond liner.