LOL
Thats exactly what I had in the fish-house.
Make a box pond frame from 2x2 and line it with chipboard/plywood. line this with polystyrene then insert your liner be careful with the corners let the water take it into the shape. pull gently to get rid of most of the creases. (although you wont get rid of them all especially the corners (such is the nature of the shape).
Trim and make a nice mited edging to hide the liner.
Blue liner is best as you'll be able to see the fry/fish.
Regards
BigC