Homemade Laminar Flowhoods?

[please_post at noreply.com]

It seems all the links I find online about homemade laminar flow hoods are on the
order of 4 or 5 years old. I'm finding more and more small HEPA-type wind boxes
in stores, some very small ones for as low as $40. I'm not exactly a handyman,
so I wanted to ask:

Does anyone know if these larger ($160 - $250) home models are pure enough and 
with sufficient flow to attach to a box and make a reliable small work bench, or 
would I be better off buying a blower, a filter, and doing it as referenced, say, at:
www.vengers.com/culture/hood.htm ?

I built a very small prototype (about a cubic foot box with a $40 HEPA air filter
behind it), and exposed a dish of MEA to it for an hour.  It came up contaminated
in a few spots (red-bread-mold, something penicillin-ish), but not horribly unclean.
How efficient is a good hood? Can you expose a plate in a good hood for an hour and 
expect it to remain clean?

TIA for any insight!