Hey why don't you just get a bunch of blocks and build it above ground and put a liner in it? Build it to the size and shape you like.
Laying the felt underlayment. We had two pieces of felt. We had to figure out the best way to install the pieces so they covered the entire area.
This is another shot of the felt underlayment installation.
This is the 18' x 10' sheet of 45 mil EPDM rubber liner. It was surprisingly heavy.
Filling the pond. This is the time to fix any creases in the liner.
Here, the pond was filled, and the cap stones were in place. The next day we placed the field stones around the outside edge of the pond
We finished placing the rocks along the edge of the base of the pond, and covered the area near the grass with mulch.