I am building 2 small ponds to hold excess water, we are planning to use bentonite clay with the intention of sealing the pond 70-80% with some seepage to benefit the surrounding vegetation.

The soil is mostly sandy loam, a tiny bit of clay, but very permeable, and the sides quickly erode and change shape when the water pours in.

One of the ponds is 3m in diameter, and the other is a bit more oval (4 meters long, 2 meters wide), but we do not have access to any heavy rollers and want to use the clay to line the ponds by hand. Any advice (regarding the shape of the ponds and the application method)?