I’ve been making a few boxes lately and have at least one more to go. I saw this technique for fine tuning the bottom or top edges of a box on a YouTube video about guitar making. In the video the luthier had a large sheet of sandpaper glued to a piece of plywood. After creating the sides of the guitar body, he would rub the edges of the body (sans bottom or top) on the sandpaper to even out the edges completely.
Since my joinery is far from perfect, I’ve been using this method on a smaller scale to even off the top and bottom edges of my boxes. I use a piece of 60 or 80 grit sandpaper, holding it firmly with my left hand while I use my right to scrape the box body away from me until the joints even off.
It’s perhaps not exactly craftsman-like, but the results are what I’m after. I’ve been usng the same system to even out my mitered corners, by running the sides on the sandpaper as well. Final sanding on the sides is done with a random orbit sander.
-- Ni faru ion el ligno!