Here's the hard part...just lining the sides with bentonite would be good, theoretically. But, if you miss a spot, there will be a leak.
If willing to dig to clay, go another few feet and use the clay. It won't cost nearly as much to use what you have as it does to bring in the bentonite, all things being equal. If you have to move quantities of sand to get to clay...well you gotta move it, anyway.

Teach a man to grow fish...
He can teach to catch fish...