If your going to go to the trouble of building a small pond with a liner I would atleast use 2 of those billboard liners, the price is good and it's cheap insurance.
Also if you can put down some sand and make the bottom as rock free as possible, taking your time in the preparation can save you alot of headaches.
I put 3 layers of cheap 6 mil plastic under my main pond liner, the backyard pond is only 8 x 16 but I didn't want to worry about leaks.