With the right materials and some careful work, you can create a minimalist, custom case for your every-day-carry essentials. This case has a hook-and-loop closure, a loop for a carabiner, or wrist strap, and a front pocket for a couple of cards.

Step 1: You Will Need:

Materials:

Duct tape
Transparent tape (packing tape, or even "Scotch" tape will do)
Cardboard
Adhesive hook-and-loop tape
Credit cards (or cardboard cut like credit cards)
Flexible plastic sheets (in this project, I used a really tough shopping bag)
Carabiner, or split key king

Tools:

Ruler
Scissors (for cutting cardboard and plastic)
Utility knife (for cutting duct tape, and packing tape)
A big, clean working surface that you can stick, re-stick and unstick tape to (Hint: no hardwood dining tables)

Step 2: Create a Fake Phone

Put together your fake phone. I measured and cut some cardboard to be a little bigger than my phone, so that my phone can slip in and out of the case easily.

You're going to be winding tape very tightly around the cardboard, which is quite punishing. Another good reason to leave your real phone out of the process.

Step 3: Cut Out Parts of Phone Case From Plastic Sheet

Cut out these plastic pieces:

Main body: should be wide enough to wrap around your fake phone, with overlap, and long enough to fold over the bottom
Card pocket: long enough to fold over a couple of credit cards, and one inch wider
Biner strap: twice the height of your phone case
Outside flap: again, twice the height of your phone case, minus a couple of inches

I built my case for an HTC phone, and a couple of cards. Your measurements may vary.

Step 4: Prepare Main Body and Pocket Piece

In each of these steps, you'll be securing your work to your working surface with packing tape. Since packing tape is thin and adds very little bulk, you don't have to worry about removing it from your work. You'll cover it with duct tape, anyways.

Center the pocket piece with the main body, and line up the top edges. Bind both edges with duct tape by laying tape halfway over the edge, and folding the rest over. This will be the inside of your phone clutch.

Bind the bottom edge of the pocket piece with duct tape, and fold it up towards the top edge. Make sure this edge falls a little under the top edge. Place some cards in the pocket, and cover with duct tape. Make sure you don't accidentally tape your pocket shut.

The cards are there to give your pocket the right amount of tension. Without them, it's easy to make your pocket too tight for cards.

Flip the main body over. Bind the bottom edge with packing tape. Then lay some duct tape over the fold line. The placement of this layer of duct tape will be important later.

Step 5: Wrap Your Fake Phone With the Body and Pocket

With the pocket facing inward, wrap your main body piece around your fake phone, and tape in place with packing tape. Make sure the pocket is centered over the wide "front" of the fake phone.

Fold the plastic over the bottom. Tape the fold down towards the "back" using packing tape. The duct tape on the fold line should cover any bare plastic. Hold the fold in place with packing tape.

Ugly, isn't it? It's okay, you will cover that up later.

Step 6: Prepare Front Flap and Biner Loop

Start covering the front, and back of your flap with duct tape.

Cover the biner loop with duct tape. Wrap it around the plastic lengthways. Plan it so that you have one "good" side with no visible tape seams.

Step 7: Attach Front Flap and Biner Loop

Dry-fit the flap to your work, and decide where your want the edge and fold to fall. Once you've established this, tape it down with packing tape.

Dry-fit the biner loop, then stick down ONE END with packing tape, leaving the other end loose. Make sure the "good" side is facing your fake phone.

(Do not try and stick down both ends of the biner loop in one pass. It will not hold. I learned this the hard way.)

Secure both the flap, and the one end of biner loop with one layer of duct tape. Make sure the overlap is at the "back" where it should be less visible.

Fold down your biner loop so that it actually forms a loop. With the "good" side now facing you, wind another layer of duct tape around your clutch.

Step 8: Finish Mummifying Your Phone Clutch

Carefully cover any remaining exposed areas with the last layers of duct tape. Keep the overlaps thin, so that a wide surface area of each layer of tape is visible. It just looks tidier this way.

Place your adhesive hook-and-loop tape on the body, and flap of the phone clutch. Close the flap.

Step 9: Blocking Your New Phone Clutch

Before you start using your phone case, give all those layers of plastic and adhesive time to settle. Leave the fake phone in, and wedge your work between a bunch of books on a bookshelf. Leave it like this overnight. Don't skip this step. It makes the difference between a quality finished project, and a cardboard box you covered in duct tape.