When we had to deal with
founder on my daughters new pony , we got a childrens wading pool
filled half with sand and then put
the water over it enough, that
her feet where covered. It was so much
easier then trying to get her
in 4 buckets of water at once!
Plus, the wet sand cushioned her feet!

Note:
This is something you would need to plan ahead for. It would be hard finding a childs plastic pool in the winter!