Not sure how many CFMs a shop vac pulls, but at school we made a flow bench for a much smaller engine just using the suction of a shop vac. The cabinet for the flow bench was just particle board and lexan with a nice seal at all the joints. I can't imagine it's much harder than making a sub enclosure, but you'd need a vacuum source. I somehow doubt that the shop vac would work for the 3.3 boxer 6