In the B&W photo you are referring to, will notice there is no bridge. It is ready for finish, but I am running low on TruOil and need to save it for a couple of custom ukes that I am working on. I don't want to go to Wal-Mart for more Oil until this covid-19 settles down.
There are a couple of tricks to making a finger joint.
First, you don't want the sides to be too thin. They need to be at least an 1/8" thick.
Second, Keep in mind that on the top edge of each side piece there is a notch on the left, and a finger on the right. If you screw this up...there is no saving that side piece!
Third, put all 4 sides in a vise and carefully lay out the fingers. 3/16" to 1/4" work pretty well. Shade the notches with a pencil. It is easy to get confused and make a cut on the wrong side of the lay-out line.
Fourth, you
MUSTmake your cuts on the waste side of the layout lines (the shaded notches). The fingers have to be preserved and uniform in width.
Fifth, use a back stop behind your bandsaw blade because you need all the slots to be an even depth. Make the depth a little deeper than the thickness of the sides. Clean out the notches by making thin cuts between the lines with your bandsaw. Once the sides are glued together, you simply round off the overlapping fingers.
Lastly, it is pretty easy to adjust the fingers to the notches with a utility knife, making ultra fine slices to get a good fit.
The joints are pretty forgiving. You may have to wedge in a filler slice here or there, but they sand out and become invisible.