I added 3/8” wide by 1/4” deep dados to the upper and lower stretchers for the slats.
I then glued in some 3/8’ plugs between the slats. After the glue dried, I trimmed the plugs off with the table saw, then sanded them flush with the horizontal belt sander. Could have used a router to trim the plugs flush too. This works better I think than mortise and tenons for the slats.
Test fit the lower shelf to check the fit. I plan to pre-finish the shelf before assembly.
Glue up of the sides
I pre-finished the slats with wipe-on poly before assembly.
Finished side assemble.
The next photo is some maple I purchased for the drawer sides. It has been sitting in a barn for many years. It is a bit funky looking, but don’t know if I should use it or not for the drawer. You never know how wood will turn out until you mill it down and discover what is inside.