I have made my own easels using 3/4 black foam core. The material is very flat and a easel for a 20x30 weighs about a pound (maybe less). A metal bladed easel is also flat, can weigh a lot and the blades can go out of square pretty quickly.

On the edge of the board I glue a slotted lip. The lips are 2" strips of 3/4" foam core with 3/4" slots at 8" (16x20) 10" (20x20), 12" (20x24) and 15" (20x30). In the slots I place 20" X 3/4" x1" 'blades'. On bigger enlargements where flatness may be a problem I use smaller pieces of 3/4 foam core in the inner slots to hold the paper flat.

This arrangement has worked ro me.