I have used the back of the backhoe bucket to compact a couple of core trenched below grade after stripping the topsoil (in excellent clay soil). They have not leaked, but I did not take shortcuts on the tamping which takes a lot of time. I used a sheepsfoot above grade. Do you have the topography to install a drain in the bottom of the pond (I used a 3" schedule 40 PVC pipe for my last pond and put a valve at the bottom of the pond in case needed later). Having a drain let me get plenty of clay from the basin to over-build the dam. I tried to explain the advantages of a steeply sloped pipe above ("One additional thing to consider is refitting you pipe with the exit end as low as possible..."). This allows a pipe to carry as much water as a larger pipe set with little drop when full because the water in the pipe pulls more water in like a siphon when the drop increases. The anti-vortex plate keeps the pipe from pulling air which decreases the pipes capacity. Having a drain gives you time to work slower with existing equipment. A dozer is nice for moving soil out of the bowl if things are not too wet. You also get a deeper pond. If you zoom in on the far left in the pic below, you can see the hole I dug that has the drain pipe in the bottom.
