We measure the inside and went to Home Depot. We bought a piece of wood and metal brackets.
N put the brackets in - two on each side.
Then I filled it with office supplies!
This is a blurry picture but you can see two boxes in the corner. The stacks of paper are scraps of paper with "Notes" and "To Do" for notes. I keep a stack of each on my desk and use them constantly - I just printed on the backs of used paper and cut the pages into fourths.