I use a stack of flat rocks for both inside and outside the 12 inch tall goat water tank we use for the duck pool. Easy to put in and take out for cleaning day.
The pool is now permanent and about 2 1/2 ft deep so it would take multiple cinder blocks to get it high enough for the babies and i need to be able to take it out at night for my resident mallard couple to sleep in the pool at night without hitting anything