Calculated, cut and glued the extensions onto each mounting board. These boards will eventually have another very thin layer glued over the top of them so using extensions instead of completely re-doing them is reasonable IMO.
Sizing them up to the master board.
Time to make the boxes. These tops are identical to the mounting boards except for being 1/8" thick instead of 1/4". When I finished cutting the boards I noticed that the widths were off which is very unusual. It must have been a bad day for the laser because these boards are suppose to be exactly 3" wide from the factory.. I fixed it by tacking on a 1/64" strip shown here before trimming.
To make my 2" tall box sides I start out with 1/8" x 24" stock that are slightly wider than 2". My carpenters square is exactly 2" wide so....
I clamp the boards to the square on one end.
And the other.
Using my Olfa razor knife I shave the wood down almost to the steel of the square. Using 60 and 100 grit sandpaper I finish hand milling the edge down to the metal.
The result is two 2" wide boards with sharp edges.
I then cut out the box sides by measuring each side of the mounting board, adding an 1/8" and then adding a skosh. 4 long sides and 4 short sides for two identical boxes.
The sides will be assembled in what I call a "pin-wheel". Here I have loosely arranged them to hopefully better illustrate the pin-wheel. Each end is facing clockwise.
To assemble I first lay down the mounting board and loosely place on top two 1/2" spacer blocks.
On top of that goes one of the box tops.
I assemble my pin-wheel with a rubber band to hold it all together. This process usually resembles some sort of clown act until you get the hang of it. Here I have decided to go with a counter-clockwise rotation. If I needed extra strength I would just add another layer of wood using a clock-wise pin-wheel rotation.
I next add another level of 1/2" spacers.
And the remaining box top.
I use yellow carpenter's glue applied with a small brush. The block is shown prying apart the seam to expose the gluing surface and is really just illustrating what my fingers are doing during the gluing process. Note that I'm only gluing the edges of the sides. The box tops and mounting boards are not being glued...intentionally.
Add a multitude of rubber bands and after about 5 minutes of drying I flip the entire assembly over to expose the mounting block. Here you can see the "skosh" I added earlier to each corner. These will later be sanded down.
I then push down on the center support and break any bonds that excess glue is trying to setup. Wait another 5 minutes and repeat. Let assembly dry for three hours.