I like the design of the "craigslist" washer pits. The closest thing to "real" washers pitched on dirt. If the pits were to get a lot of use, I would use metal pipe, 3 1/2" OD. I would also make the a little shallower so that you wouldn't have to carry around so much dirt.
I have two 6" lengths of 3 1/2" OD metal pipe pieces. When we go camping or to a party, I just drive the pieces in the ground 21' apart and dig the dirt out of the holes. Pretty fast and simple washer "court".
I have two 6" lengths of 3 1/2" OD metal pipe pieces. When we go camping or to a party, I just drive the pieces in the ground 21' apart and dig the dirt out of the holes. Pretty fast and simple washer "court".