Before building our lovable loo I acquired 4 identical used food grade pails with handles and lids knowing that I could get more from the same source. This gave me the measurement I needed to build the frame the correct height so the pail protruded about 1/2 an inch above the finished frame.
I was surprised when I increased my number of pails to notice that the new ones were not all identical in height. About one in four of these pails is 3/4 of an inch shorter than my original four pails. Luckily I built for the taller pails so to use the shorter I keep a piece of scrap 3/4 inch plywood in the frame to put under the shorter pails. It's nice when the luck you have is good, not bad.
Life is too short or my list is too long, not sure which.