I bought a Old Hickory Ohio Sticker knife. It is made of good quality steel and is less than $15, but it needed a case. Like most kitchen knives, it came with a card stock sleeve. I decided to modify that sleeve into a sheath. I started by drawing the outline of the tip on the card stock. Then I trimmed it a bit longer than I needed.
Roll out some duct tape and place the ugly side of the card stock down first. This will become the back side of the sheath and will rest against your leg unseen.
Step 1: Wrap the Tape From the Back to the Front
After the back side of the knife is attached to the card stock, flip it over and cover the front side. Be sure to press the edges firmly to create a tight seal. Do NOT cut the tape yet. The next step will create the belt loop. This will be one long continuous relatively strong section of tape. Do NOT cut it until after the loop is finished.
Step 2: Create the Belt Loop
Measure the size of the loop with your belt and allow for some extra room. Wrap the tape back to the front. In the first picture, I have some leftover tap that was not needed. I pressed the tape down firmly then trimmed it close to the knife opening. The second picture shows my double check on the belt sizing.
The third and fourth pictures are hard to understand, so I will explain them. I started my second section of tape on the belt loop right where it meets the knife slot. I covered the entire belt loop again giving it four sections of tape total. The last picture here still shows that the tape is attached do the roll and ready for the knife to be flipped over to attach the belt loop to the back of the knife sheath.
Step 3: Attach the Belt Loop
The first picture shows a very long belt loop. It is looped and taped along the entire back side of the sheath to securely attach it. Only then is it cut from the roll and trimmed to fit. I trimmed off the corners to make it look appropriate for a double bladed knife.
Step 4: Front and Side Views of the Case
Here is the finished case in the front and side view. It was a cheap and effective way to create a useful case for my knife. This method should work for any knife.