do you need a filter for the pond? are you going to be growing koi or tilapia in it? There are several large koi ponds around my area that only use "fountain" style pumps for aeration and no other filtration. When they're that big at 56000 gallons, they start up their own ecosystem pretty quick. I'd plant it with lilies and marginal plants and let them do some of the work.
As for making the thing hold water, I don't know what kind of clay would help, but nothing beats a liner. I know it's expensive, but I don't know what else will hold water for you... clay is porous, so even if it's some nice clay, it'll still absorb and pass water through it.