Here is how I did mine. I took it one more step.
I downloaded the pattern for the boxes from here:
onto cardstock. One box per sheet. Wish they made cardstock legal size, but I haven't found it yet.
Don't cut them out, just trim away the excess. Then I ironed fusible web to the side WITHOUT the pattern. Don't use steam or your paper will curl.
Then when that was done, I laid the cardstock with the fusible side down onto the BACK of the fabric.
I press from both sides. I used my pressing cloth and my teflon sheet. Teflon sheet to take the excess fusible web
When it had cooled, I took it to the cutting mat and using my ruler and small rotary cutter I trimmed away all the straight lines.
I bent the box on the lines and then I glued the longer side with tacky glue.
I wonder what else you could use these for. I'll have to put my "cap" on and give it a think!
You can click here for a PDF of these instructions.