I have an 18 inch professional JBL driver in a 7 cubic ft home made enclosure. Will give peaks up to about 125 dB dwon into the upper 20 cps range. The driver is about $500 ish and the box costs what you want to put into it. Figure $150 plus whatever finish you want to do. Veneer works nice. Suggest at least 1 inch high density particle board with 2x2 hardwood inner frame and bracing. Ported with a 6 inch diameter PVC pipe. Will take close to 1KW peaks and 200 watt steady state. Watch the windows though.
I use just my 15 inch jbls with horns for most of the time thuogh. Plenty of output down into the upper 30 cps range and the compression driver horns have excellent clarity. If you want to shake the room though, the 18 is the answer.