Powerheads work best... but I've also heard some people on Reefcentral use saran wrap. Turn off your power heads, put a sheet on the surface then lift off, the oil should stick to the plastic. Dunno if it works, I've never tried it. Could be whoey
If its only for a short while I'd just use powerheads, any time longer I would just get a used prism off the Buy sell forum or ask a fellow reefer to lend me an old protein skimmer from one of their earlier systems (you know we all have it.. old stuff kicking around from when we didn't know any better.)