Our team actually took two shop vac impellers and mounted them on a custom turned steel shaft from the lathe. We then used the built in housing from the vacs to enclose the impellers. We then built a wooden frame to mount the impeller housing and bearings for the shaft. In the middle of the wooden frame, we put belt sprockets on the shaft and mounted two fisher price motors with belt sprockets as well. This gives us a VERRRRRYYYY powerful vacuum. Two fisher prices are more powerful by one cim by a factor of around 1.3. Check out this link at around 1:22 to see exactly what I mean:
http://www.youtube.com/watch?v=X_dnzebFqDk
Good luck! Let me know if you have any questions!