Making a speaker box from a cardboard box may seem like an odd thing to do. But perhaps you need a small extension speaker in your garage or by your workbench and you don't want to take a lot of time or expense to make an enclosure. A sturdy cardboard box can get the job done as well as almost anything else.
Find a suitable box. You'll want a relatively small box built of heavy stock cardboard. Corrugated cardboard is much better than single ply cardboard. A small box will tend to have more structural rigidity than a large box, so err on the side of small. If you're using a speaker smaller than 6 inches across, use a box that is roughly an 8- to 9-inch cube.
Cut an opening in the side of the box large enough for the speaker. Then cut another piece of cardboard the same size as the side panel of the box with an identical cutout. Tape or glue it into place behind the wall of the box. This will provide extra strength and reinforcement for the wall that must support the speaker.
Poke a hole near the bottom of the side of the box opposite the speaker opening and thread a piece of speaker wire through it. Make the hole just large enough for the wire.
Mount your speaker into the opening. Hold it in position with bolts and nuts driven through the cardboard wall of the box. Connect the speaker wire to the terminals of the speaker
Close the box and seal it with duct tape. To strengthen the box, reinforce the corners with duct tape as well.