Use an empty closet. It can be turned it into a vocal booth in a pinch. Attach sound absorption foam to the walls from the waist up. If you're concerned about your walls, you can glue the foam onto styrene boards then attach the boards to the walls using Velcro strips. For the door you may need to tack rubber seals around the edges to help control leakage.
Build a three-hinged wall. A vocal booth doesn't have to be a room; it can simply be the area where the sound is traveling into. In this case, that's the immediate area around the microphone.
Nail two 77-by-2-by-4-inch piece of wood to the edges of a 2-by-5-foot piece of plywood. It should be able to stand by itself. Repeat this for two more the attach to each other using hinges so that the walls can be adjusted by folding in and out. Glue absorption foam on the inside of the plywood face. Place rubber seats on the bottom of the 2-by-4 to ensure the walls don't shift and are stabilized.
Invest in a portable vocal booth. This is actually two 2-by-2-foot absorption panels mounted on an microphone tripod stand. The panels are large enough to where the sound from your voice is blocked before it can get into the room and reflect off of the walls.