I know it sounds like a task mate but there's lots of vids on YouTube how to build plywood tanks using liquid rubber as sealant it just goes on like paint lots of layers (not sure cost though need to research it in oz ) , the simple way of course is to build a raised frame with plywood sides and layers of pond linner ( basically the frame becomes your hole in the ground ) , ill try find a few links later and e mail em ya , tanks looking good though mate lovin that perch how big they get ?