I was recently in my shop building a box and my joinery didn't exactly work out as planned. The bottom of the box was slightly off kilter so I developed a simple method for flattening it. I had a granite block, that I use for sharpening plane blades, in my shop and some 80 grit sand paper laying around so I did the following:
Used spray adhesive to temporarily attach the sandpaper to the granite block.
Using light pressure I moved the box, dry assembled, in a clockwise motion on top of the sandpaper until the box sat flat on my table saw surface.
I repeated step two for the top edge of the box.
Interested in the products that I used to do this? I have included links to each one below.
If you have any questions for me on this method please leave them for me in the comments. Also, if you used this method after reading this and saw some success I would love to hear about it.