Hi Ian
We use a simple system on building sites to create temporary ponds.
Imagine the open topped box inside a matchbox and then think big.
All you need are some steel pins which were usually 3ft lengths of 3/4 in reinforcing bar, and some wooden boards, (single or double height scaffold boards laid on end) and a large tarpaulin or waterproof groundsheet.
Form a large open topped box with the timber boards and support them with the steel pins.
Open the groundsheet inside the box with the edges draped over the boards and then fill the pond with water.
Cheap, robust and easy to move around.
Paul