John, funny you should ask. Tomorrow I am getting ready to revamp my little pond made with a billboard liner. Will post pics and details. Right now I'm sitting for a minute before dinner but will fill you in more later.
I really enjoy the pond. I have had small 40 gallon contraptions for years. I always try to make sure that there are very shallow areas and moist rocks to land on for butterflies, birds, dragonflies and lizards.
The only time I ever 'clean' it is after all the leaves are off the trees and then I will just take a net and scoop out the major chunks of leaves. Also the waterlily roots can become massive so I will divide them every couple of years or so. You just have to remember that wildlife have laid their eggs in these leaves and roots so when you 'clean' you are destroying a bit of habitat. I just like to overflow a bit and I don't worry about algae or clear water. Natural ponds are seldom crystal clear. I have had fish living in a 40 gallon pot for three years until a dog climbed into it and destroyed everything. No more dog.
We have several of the billboard tarps. Very heavy. I think it could work well, as long as there is a thick layer of sand over it and no animal traffic or other pokies. However, a real pond liner is much thicker and will self-heal to a degree. Best of all, clay in the soil.