Step 2: The paper pattern
To mark a circle around the pipe, for cutting it at 90 degrees, use a piece of paper with a straight edge and a pencil. Wrap the paper around the pipe and line up the ends of the straight edge. Mark the pipe along the edge of the paper.
By marking and folding the paper you can get half-circumference distances, etc.
I put the plates 2 1/2 inches apart, center to center and cut the plate holding slots 7/8 inch wide at their widest part.
At the stage of the paper pattern, I suspected I wanted a crescent-shaped hole, but didn't know how to draw it at that point, so I did the pattern with rectangular holes and penciled in the crescent shapes later by eye. After botching one hole, I got it right and copied it for the other holes. The point at the bottom of the crescent shape helps hold the bottom of the plate firmly. If the cut was rectangle shape, the bottom of the plate could shift, changing the angle at which it is held. I wanted the holding angle to be the same on all the plates.
Not all plates are the same shape, so you will have to customize your holes to fit the plates you will be using.
Keep in mind that the plates lean forward some. Because of that shift of center of gravity, I made the front of the plate holder a little longer than the back to avoid possible tip-overs if there was only one plate in front.