Attach a piece of 100-grit sandpaper to the bottom of your electric sander. Run the sander across the surface area of your crate. Repeat with 220-grit sandpaper and 360-grit sandpaper.
Remove the dust from the crate by wiping it off with a tack cloth. Select a color for the outside of the crate, ideally a color that complements your collection of mementos. Spray-paint the outside of the crate that color, holding the spray can 8 inches away and making even, smooth strokes.
Measure the five inner sides of the crate. Cut five pieces of fabric to match the dimensions of each side. Spray the inside with fabric adhesive. Once the glue starts to get tacky, press each pieces of fabric onto its respective side.
Arrange your mementos around the inside of the crate. Once you're happy with their formation, hot glue them in place. Measure the length and width of the open side of the crate.
Visit a hardware store and get a piece of glass that is 1/2 inch longer than the length and width of your shadow box's open side. Ask the hardware store to dull the edges of the glass so it's safe to display. Place the glass on top of the center of your shadow box coffee table.