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You can make this design to size, if you know what size you'd like. Measure the longest pen. You can do this project in just one color, but if you have a second color, you can make a contrasting liner. We'll call the two colors the liner color (shown here in yellow) and the cover color (shown here in blue). If you want you can personalize it, try using different colors of tape, stickers, craft foam and pipe cleaners.  This will be the cover color tape, and it should be about half an inch or 1–1.5 centimeter (0.4–0.6 in) wider than the liner tape. You can trim the length later. Attach the liner tape strips together. Make sure that the sticky portion of the cover color tape has a half-inch or 1–1.5 centimeter (0.4–0.6 in) allowance at both sides of the liner color tape.  This will form the pocket. After folding it back in place, insert the longest pen inside the flap. You could bring the lower pocket all the way up or you could fold a strip of tape in three and put it near the top. Trim the finished product to the width of the liner color and set the strap loosely in place. Fold evenly! When working with duct tape, you can lift if for the first few days. After that, don't count on changing anything. Decide where you want the fold. You may want to leave a little extra length in case you ever get a pen or pencil that's a bit taller than the ones you have now. The easiest is to cut a straight line on a slight angle, but if you want to cut a bit of a curve, you can. In order for the sides of the curve to be equidistant, fold the top flap lengthwise in half and cut the curve through the edges while they are held together. If you curved the end of the top flap, you'll need to cut little slits in the duct tape on one side to receive it so it will stick flat. Here, there's another bit of tape to line the tab.

Summary:
Gather the pens and pencils you'd like to carry in your case. Choose two colors of duct tape. Using the measurement you have taken of the pen, cut a piece of duct tape that is 2-3 times longer than the measured pen you want to hold. Cut two other strips and attach all three of them together to form a duct tape fabric that is a little wider than your selection of pens. Next, cut strips for your liner color tape, which should be the width you want your finished pencil case to be. Put the cover and liner tape strips together. Tape the edge of the bottom of the duct tape fabric and make sure that the two colors are even. Fold the bottom end up, and crease it without sticking the edges yet. Trim the sides of the folded portion. Using the pen as the point of reference, make an upper strap or loop. Fold the sticky side flaps in and stick them down over both the bottom pocket and the upper loop. Put your pens in the pocket and check the size. Fold the top flap down. Trim the end of the top flap so that it will stick into the bottom pocket. Fold short pieces of tape over the sides and end of the flap and any exposed edges. Reinforce the bottom of the pocket with another piece of tape if you want. Put your pens and pencils in the case, fold the flap over, tuck it in, and enjoy!