Well I don't know why John's trays have such low sides and IIRC, they only have three walls. Maybe it is a period thing.
Originally Posted by schrochem
I built mine with walls that are about 3/4" high. They have a second piece of acrylic glued on the right side to form a ledge slightly higher than the level of the glass. A hole is centered in the area where the plate will rest and I smeared some acrylic cement over that surface to make it slightly uneven. So, a finger can be pushed from below and easily break the tension holding the plate to the tray.
A top view
In use the plate is placed in the tray and the tray tilted to the right. I pour the developer onto the ledge and let it level. Then I quickly tilt the tray back to the left and the developer flows evenly off the ledge and over the plate. If it looks like I might get an island, the tray allows me to violently move the plate horizontally to cover the spot without the plate flying off into space or the developer being spilled.