Determine the joke. Famous prop comedians like Gallagher and Carrot Top custom-build props to make a specific joke, such as something in the news or a play on words. Rather than making a humorous item, determine what the prop is meant to accomplish from a comedy standpoint and design it from there.
Be creative. Hundreds of different sight gags exist, and some are often considered cliche, like a rubber chicken. The most successful prop comedians use their own creativity to determine the prop. The famous prop comedian Carrot Top has challenged other comedians to design props for him, but that has never worked out. This is because his humor and creativity is unique to his act.
Use found objects and soft material. The purpose of a prop is to have it look like the real thing, but it does not necessarily have to be exactly what it represents. If the props are used for slapstick, they can be made from soft materials, like cloth or foam. Old jars and cans can also be used to make any cylindrical element of a prop.
Exaggerate. Live performance requires that even members of the audience in the back row can see the act, so props are often made larger than life. For example, Gallagher's famous Sledge-o-Matic hammer is not a real sledgehammer but an over-sized exaggeration of one, and it's much lighter than one made of solid metal. Another example are cartoon style firecrackers that can be made from emptied juice cans painted bright red with the words "TNT" or "Dynamite" stenciled on them.
Use noises. Performance comedy isn't just about sight gags and telling jokes. It also uses noises to strike out at humor. A simple example of this is the slap stick, which is where slapstick humor gets its name. Two thin pieces of wood are secured together on one end and left open on the other. When someone is hit with the slap stick, the wood slaps against each other, making a loud and shocking noise, even though the action does not hurt the person being hit.