What you're looking for is how to construct your own laminar flow hood.
One of those can be bought for big bucks, but can be constructed yourself.
This is the best guide I've found on the specifics:
http://www.fungifun.org/English/Flowhood
You want to run some calculations so the blower you are matching up to your HEPA filter produces laminar flow (100 feet airflow per minute). You also need a prefilter (a furnace filter is commonly used) to protect your sensitive HEPA filter from larger particles, and will need to take the resistance of that filter into account as well.
I've looked into this myself, but I don't know anything about electricity or blowers, so I'm holding off for now. I might just shell out the money for a prebuilt one somewhere down the line, though it doesn't seem
that hard to make one.
"The only way to deal with an unfree world is to become so absolutely free that your very existence is an act of rebellion." - Albert Camus