It's not exactly rocket science.
Find a full range 8" to 10" speaker - 8 or 16 ohms. Cut off the end of an mic cord, and connect the black wire to one terminal of the speaker, the white wire to the other terminal, and the ground wire (shield) to the speaker frame.
Mount the speaker in an open back box, or simply to a small baffle board, and plug it in. If it doesn't sound good, get one of those 600 ohm to 10,000 ohm xlr converters and try that at the mixer end. The converter is just a 20:1 transformer, so 16 ohms in will give you 320 ohms out, about what a mic pre likes to see.
