I probably wouldn't try to make an adjustable easel, I would prefer to make one for a single size of paper.
In fact, I already have!
A couple of years ago I made something to take Ilford's postcard paper. It was in two pieces and consisted of a heavy base with rubber feet and a couple of 1/4" pins and a removable part which locates on the pins and is in the form of a rigid envelope with a rectangular window cut out of the front.
The envelope part is placed on the base and it is moved to the correct place for the desired composition then the image is focused on the back surface which has some lines laser etched in the thirds positions as a guide for horizons and verticals, etc.
The envelope is then taken off of the pins, paper is loaded then it is replaced on the pins and the exposure is made.
This gives a repeatable result with even borders.
I have not tried to make one for anything larger than the Ilford postcard paper yet but I may have a go at one for 8" x 10"
I will post some pictures if I can find them. EDIT: I have found some of the top part.