Now, you will sew (or just stick, depending on which velcro you bought) your velcro on. Granted, you are be able to do this in Step 5 when you've sewn your fabric in your panels.
Sew two strips of your velcro on the inside to allow for different placement of your inserts. This is easy, and you can even sew completely through your box if you'd like, since it will be hidden in a bag. Make sure your velcro is securely sewn in.
You will also sew velcro onto the sides of your foam inserts to correspond with your velcro lines. Use a thimble!