My grandmother had this problem with her rain barrels when the water got too low in them. So, she cut up a couple of my grandfathers' fishing nets and stapled thet nets over the tops of the barrels-much to my grandfathers' horror. Then, in the cow troughs, she cut tree branches up and put them in there, overhanding. The cows could scratch their rumps and the frogs and toads could come and go as they pleased!