While browsing Instructables, it occurred to me that, of all of the guitar straps on the site, I was only able to find two made of duct tape, and the single adjustable one relies on string to adjust. So, I decided to remedy this situation by creating an adjustable strap, made entirely of duct tape (apart from the neck connecter).
Duct tape (of course)
Secondary color (optional)
This is based on the strap on my dad's guitar. It consists of two pieces; a thick strap ,which is the main part, and a thin strap, which is responsible for the adjustability. We'll start with the thin strap.
Step 1: The Thin Strap
Cut one strip about 13.5" in the primary color (in this case, black). If you attach your strap to the body and not the neck, make this a few inches shorter. Fold it in half lengthwise, so that it's half the width of the tape. Slide a finger along the edge to remove air bubbles. Do the previous three sentences twice. Lay a new strip of tape between 8" and 12.5" on your work surface. Place one of the folded strips on top, with the folded edge along the edge of the shorter piece. Overlap the second folded strip over the first by a half inch. Fold the short strip over the first two.
Step 2: To the Ends of the Thin Strap
Make a square of tape by taking a strip of tape about 4" long and folding in half the shortwise. Take another strip about 4" long and wrap it perpendicular the first square. You should have a 2" square of tape four layers thick. Lay 1" of the thin strap on top of the square, and starting at the edge that the thin strap is on, tape it on with another 4" strip. Add a fouth strip running perpendicular to the last one to strengthen the square. This square is instrumental in the ability to adjust. Make a 1" mark across the middle of the square just below the end of the thin strap (See picture 5). At the other end of the strap, wrap a short (4" or so) piece of tape around the top 2". Set this strap aside for now.
Step 3: The Thick Strap
Now we'll make the thick strap. This one is twice the width of the first thin strap (It will be the same width as the tape) and about 25" long. Cut two strips of tape about 13" long and put them together sticky side in. Be careful while going this; duct tape like to stick when and where it shouldn't. Like you did with the thin strap, make two of these, then tape them together the same way you did the thin strap.
Now place a strip halfway on the edge of the strap, as in picture 1, and fold it over. Do this all the way up both edges. Reinforce the ends like you did on the smaller end of the thin strap. About 1.5" from the end, make a mark just longer than the width of the thin strap. Do the same 1" up the strap, and another 1" from that, and continue until you've made 8-10 marks.
Step 4: Cutting
On the other end of the thick strap, punch a hole equal distance from each side and the end. Keep punching until the hole is big enough to fit over the strap pin of your guitar. Punch a small hole about 0.5" to 1" from the small end of the thin strap, and one more 1" below. If you prefer to attach one end to the body of the guitar punch this hole big enough to fit over the strap pin.
Cut a out the mark on the square end of the thin strap. The thin end should be able to fit through the cut. If it doesn't fit, just keep cutting until it does. Cut the marks on the thick strap. After you've cut all of them, check that the thin strap will slide through easily.
Step 5: Decorate
The strap is technically done now. If you like it like this, skip to the next step. For those of you who can't stand monochromatic-ness, now is the time to adorn that piece of duct tape that enables you to play your guitar rather than spending the rest of your life in a fruitless quest for a levitation charm (but if you find one, let me know!). Diagonal stripes of a bright color look rather striking, and I imagine chevrons would also look good. If you do something else, post it in comments. Basically, make it look interesting. I did stripes of different sizes on each strap, but you're not making my guitar strap, you're making your's. Make it how you want. If you're going with stripes, read on! If you're going with chevron, read on and modify!
In your secondary color, place a strip of tape across either strap at an angle. The end should stick out past the edges of the strap. Then do another. And another. And skip several inches and do it again. And save the world from the scourge of meatless hamburgers. And go back to working on your guitar strap. My cutting has handy-dandy lines that make putting on parallel pieces of duct tape very easy. If your's doesn't eyeball it, and it'll still look good. Then flip the strap over and put more pieces of your secondary color across where the tape is on the other side. This is why it sticks out, so that you can line up the tape. After you've put on all the stripes and chevrons you desire, cut off everything that sticks out and cut everything that covers holes or slits. Those were cut earlier because it's a lot easier to cut fewer layers and then recut.
Step 6: Putting It All Together
By this point, you may be wondering how this works. Go get your guitar and put the thick strap on the strap pin at the bottom of the guitar. Slide the thin strap through one of the cuts on the other end of the thick strap so that the square catches (picture 1). When the length feels right, fold the square up and thread the other end of the thin strap through the slit in the thick strap and the slit in the square (picture 2-3), then tighten everything. Thread the thin strap through the rest of the slits until you get to the end. Tie the thin end to neck (or attach to the strap pin). If you have string that matches, use that. By the way, JonathanCM has a great 'ible on strap locks that look great with this strap.
If anything I said doesn't make sense, ask! I'll try to answer your question, or update the 'ible if I missed something. Also, I would love to see pictures.