me and my cousin made one for 35$.

2$ -- 4' long piece of 2" PVC pipe for Barrel
2$ -- 2' long piece of 4" PVC pipe for Chamber
5$ -- PVC primer and cement set
2$ -- 4" to 2" PVC converter
12$ -- 2" ball valve (big splurge)
2$ -- 4" cap
5$ -- air compressor fitting, and bushing
5$ -- 50 lb bag of ...