The cradle shown is built from two sheets of black Styrofoam posterboard which is sold at the local stationery store. Black focusses attention on the specimen. Alternatives to black Styrofoam posterboard include stiff plywood or cardboard painted with non-reflecting black paint.
Whatever construction material you use, it needs to remain flat. It must also remain in place throughout a work session, possibly by placing it between heavy bricks or, if there is a permanent imaging table or desk, by clamping, screwing, or gluing the cradle to the top surface of the table/desk.
Instructions for building a cradle are here. Attaching a ruler to one of the walls guarantees there will be a ruler in every specimen-image. The cradle shown uses an inexpensive flat plastic ruler. An alternative is to print one using this file following these instructions. You can use a graphics program to change the name of the institution before printing. After printing, check that the printed ruler is accurate before gluing it to the cradle.