A guitar capo is a device used to raise all the strings to a higher pitch by pressing a bar across the neck behind the frets. This useful device is one item that is so simple you hate to pay the $15 to $30 for it in the music store. Other than being frugal, sometimes, you just do not have a capo on you and you need one for an impromptu jam session.
Pencil and Rubber Band
Any item you write with will make a guitar capo. A simple pencil and rubber band will work well as an emergency capo. Simply lay the writing implement across the frets and string something stretchy over one end and behind the neck to the other end so the pencil is pulled down tight across the strings. You can use a handful of rubber bands, a wide rubber wrist band or any kind of stiff stretchy rubber material. Elastic will not work because it is not strong enough to pull the pencil or pen down hard so that the strings will not rattle. You can also use a large permanent marker. The plastic body is soft enough not to damage the strings.
Other Stretch Capos
You can also drill a hole lengthwise in a thick piece of a hot glue refill and push a heavy nail through it. Make it a little longer than the neck of your guitar is wide and use a big nail that is wider than the neck. It should stick outside both sides of the hot glue cylinder. This gives you something to wrap the rubber bands or rubber strips around. As with the pencil/rubber band capo, simply lay the hot glue/nail assembly over the frets and attach the rubber band(s) to one end. Pull the bands under the neck and loop them over the other end of the nail so that the glue cylinder presses the strings down behind the fret. The glue cylinder is sturdy stuff, does not damage the strings and the nail keeps it stiff and straight.
A simple screw-down capo can be made with a cabinet door handle. Get a U-shaped cabinet door handle with legs shorter than the depth of your guitar neck and a crossbar that is a little wider. Cut a piece of hardwood or metal (hardwood is easier on the guitar strings) just a little wider than the neck and narrower than the distance between the narrowest fret you are going to capo. Drill holes the width of the legs large enough for the cabinet screws that came with the handle. Then, simply put the piece of wood across the neck behind the fret you want. Put the cabinet door handle behind the guitar neck. Put the screws through the holes in the hardwood piece and into the cabinet handle and screw it down tight. You will need to keep a screwdriver around, but this makes a stable capo if you do not need a quick change out.