And here's how I did it:
Start with a wooden crate (you could also use a rubber bin or a sturdy cardboard box). This one was my daughter's, painted to match her room, but she doesn't want it any more.
Cut a piece of fabric large enough to drape over the crate on all sides, with a little extra to tuck over the edges. I bought forest green sheeting fabric (wide as queen size bed!) to make my tablecloth and more.
Flip the crate and fabric over, tuck the fabric nicely at the corners of the crate, then tape the extra fabric on the inside. Flip it back over, and it's ready to help improve your display. And a bonus is that I can haul some books in the crate to the sale.