It is super strong and sturdy because instead of using nails to assemble the frame which is often the temptation when building smaller projects like this, I used
pocket hole screws made easy by the Kreg Pocket Hole Jig.
Not exact matches
These frames were
made from scrap wood (2 x 1s), my Kreg Jig (using 3/4 ″
pocket holes and 1 1/4 ″
pocket hole screws), and a little Minwax Dark Walnut stain.
Note that you will likely have to dry fit a few times and
make minor adjustments so the shelf will fit before
making 3/4 ″
pocket holes and attaching with wood glue and 1 1/4 ″
pocket hole screws.