You should begin the construction by selecting a filter and a blower.
The filter size depends on how big you want to have your working space.
The smallest filter size you can reasaonably use (for home scale cultivation)
is 1 ft X 1 ft, better though 1ft x 2ft. The filter on the right is
a 1½ ft x 1½ ft HEPA with 99.995% removal efficiency for particles down
to 0.3 micron.
When you decide on the filter, you have to match
a suitable blower to this particular filter.
A possible combination would be:
Part Number: HSP924125
Actual Size (HxWxD): 24x12x5-7/8
Rated Airflow (SCFM): 300
Initial Resistance ("w.g.): 1.0
You should mount a good, thick prefilter(furnace filter), since this
blower is a bit overpowered.
The design in the pictorial uses an in-line
centrifugal fan, another good option (actually the one most used
for flow hoods) is a squirrel
cage blower (also called shaded