We're breaking down the trim box today.  These instructions will work for a box of any dimensions, but I'm in the middle of this particular project, so you get the ones for trim!  These instructions are meant for personal use.  Yours and yours alone, dear People.   

Supplies:
Fabric.  Two kinds.  I'm using cotton canvas, and quilting cotton.
Heavy weight interfacing-- I'm now using Bosal products. 

Cut Fabric. 
My finished dimensions will be 10" long, 2 1/2" wide and 31/2" tall.  In order to get those I'll add a 1/2" to each measurement to allow for 1/4" seam allowances. 

Sides: 10 1/2" by 4" (2 of each fabric.)
Ends: 3 by 4" (2 of each fabric)
Bottom: 10 1/2 by 3" (one of each fabric)
Bosal: 25" by 3 3/8" (one)
(You can also piece the interfacing together in side sections.  I do this for big boxes, or when I don't have a long enough piece.  You need to use finished dimensions (minus a little wiggle room) to cut out the sides, and then join the pieces side-by-side with no seam allowances using a zig zag stitch)

Stitch together the sides of the box, right sides together using a 1/4" seam allowance.  Backtack at the top and the bottom.  Your stitch line should extend from the top of the seam down STOPPING a 1/4" from the bottom.  This will help you get the bottom of the box on.

Take the bottom piece and line up the long sides together.  Begin stitching 1/4" from the edge of both pieces and stop stitching leaving the same 1/4" at the end of your stitch line.  Repeat on the other long side.  Sew the ends together last.  the 1/4" you left will let you swivel the box ends together and get a cleaner corner.

Repeat these steps with the lining with ONE exception.  Leave an opening for the interfacing to go through at one edge.  So.  Start at the top and sew down 1" backtacking at either end.   Leave an opening.  3/4" from the bottom to another short stitching line, stopping 1/4" from the bottom, backtacking at both ends.

Okay.  You still okay?  We're nearly there.  Iron as best you can, both the outside and the liner.

Turn outside piece right side out.  Turn liner inside out.  Put inside each other with right sides together lining up all corners.  Have the lining Poke out a little over top the outside pieces.  Pin together if you're feeling a bit squeamish.  If one side is longer/shorter you can ease the two pieces together at the corners.  You can also stretch the fabric a little as you sew, to make up for any deficits. 

Sew the top edge together all the way around.

Pull the outside through the big hole you left in the lining pieces and get it all straightened out.

Bend your piece of Bosal (or other) intefacing according to the dimensions of your finished box (it's a good idea to measure your outside piece at this point, just to make sure you've got the right measurements.  Bend it in half and insert it through your lining piece.  You're going to have to get a little rough with it.  That's why you did all that backtacking.

Smooth down the top edge down over the sides of the interfacing.  Pin together.  Now, the interfacing is going to be unyielding, but your fabric will have some give.  When you stretch out  your fabric and pin it through the lining you'll know what you're dealing with.  You're going to think that you have all sorts of extra interfacing but chances are good you'll have just enough.  (You might end up with a tiny gap at one of the corners where your lining doesn't quit reach.  Or you may end up with a little overlap.  If you keep your open corner lined up with a free edge of the interfacing, you can reach your scissors in there and snip off any extra.)

Start at the open edge of the lining where the interfacing meet and begin top stitching all the way around the box.  I used my machine today, but I prefer to hand-sew the top stitch in.  The machine tends to jump around a little. 

Now.  Slip stitch the opening in your lining closed. 

For clean edges, go ahead and iron in a crease at all four corners.

There are so many ways of embelishing these. You can embroider some fun stuff on the outside before or after you put it together. I sew a panel pocket on my big boxes  before I put the sides together.  And there's always applique.  Never thought I'd be using that phrase. 

Now, if you're saying "Girl, you are Crazy, that is a lot of work.  I would just like to BUY one."  Well, I'm working on it.  I'll let you know.  Until then, go forth and make boxes for the good trim keeping people in your life.   Problems?  I'll try to answer questions in the comments.