Step 4: Internal clean up
Once you make your cuts, your speaker should be in two clean cut pieces as is pictured here (yours probably won't look nearly as good as mine, but whose fault is that? Hmm??)
Speaker boxes are held together with goopy epoxy-like glues and their insides are never intended to be viewed by anyone, so you'll probably find a lot of glue globs and what not stuck to the box's inner side walls. Take a chisel or knife or whatever and scrape that stuff off, and get rid of any other stuff you might find in there while you're at it (for structural stability purposes, a lot of speakers have little wooden blocks glued into their corners on the inside).
If any chunks of wood have come out with the glue globs, or if you had a missed cut or two when you were slicing out your door, fill in those errors with either some wood filler or some epoxy putty (I recommend J&B Waterweld, as it's the best stuff on the market). After that, sand the inside of your box until you're pleased with its overall smoothness. For the record, I was chasing after Billy Dee Williams level smoothness, so I had to sand mine for a LONG time.