Build something like this.
However, modify it to just bolt the wooden top to two buckets underneath.
This does mean you'd have to drill a hole in each bucket, so they'd no longer be water-tight. But it does allow the bench to pack down to a very small volume, if you already have a third in-use bucket that you can nest these under.
An alternate way to ensure a good connection between the top and the buckets — Saw off the bottom few inches of two buckets. Permanently attach these bucket "stubs" to the top boards. Then make these stubs stack against two normal unmodified buckets.