After planing down the scrap board of curly cherry I found lingering in the garage, I milled the parts for the base of the 4×5 insert. The wood ended up just a little over 5/8″ thick; enough for some stand-off of the holder. I laid out the widths needed from the plate back and ripped parts from the board. To the rails, I milled a small tongue to fit under the rails on the plate back. They fit!
I ran a groove on the insides of the rails for the tenons on the stile ends. I did this using the tablesaw – run the groove in approximately the center of the rail, flip it end-for-end and run the groove again. Results in a perfectly centered groove.
I set the horizontal router table for approximately the tenon thickness needed for the groove and start adjusting the height of the bit, cut after cut, in tiny increments. This allows me to precisely fit the parts. However, I always stop a little short and finish up with hand planes (in this case, a Veritas shoulder plane and a Stanley Sweetheart #18):
After adjusting the fit, I test the insert out in the back:
Confident everything fits correctly, the assembly gets slathered with poly glue and will sit in clamps for several hours:
After the glue is cured, I’ll clean up the foam-out, mill a small rabbet in the end of the insert to allow it to sit over the end retainer strip on the plate back, and layout the cuts for rabbeting for the ground glass holder.