All this chatter about porti potties and boxes for same, spurred me on to go outside and take pictures and measurements of a unit I built about 5 years ago and has served us well all these years.

  I used scrap half inch plywood from another project.  The bottom is 16" wide by 17" back to front.  The sides hieght is 11" and with castors (4) and the top, the unit sits about 16" off the floor.  With the castors it rolls easily and unless it is secured by the bungy cord, it is not a good idea to use it as a step stool, although it is strong enough for that.
  It is strictly for #1 use.  Otherwise, you use it you clean it!

  Go to the link below for pictures.
  http://picasaweb.google.ca/ewieber/20100729#

  Ed Wiebe