There's a whole Resonators group here on CBN, called "How loud can we make it?" I know, it's not very obvious, is it?
Do a quick Search, and you can find several threads there on how to build CB resonators.
A similar question to yours came up over there several months ago. Several ideas, such as pie plates, aluminum pot covers, metal hub caps, colanders, tuna cans, cookie tins, coffee cans, old license plates - any of them can be used as a resonator, and many of them can be acquired for literally next to nothing. Dumpster diving has been known to yield such treasures, as havs frequenting garage sales and thrift shops.