Ok, I'll chime in even though I don't have plans.
I originally thought about building one myself at first.
The downdraft port(s) should be offset from the subject to be painted
to reduce attracting dust and what not. There's more duct work involved
and making a base with holes in it (or getting metal with holes or
screening). Since I couldn't figure out a benifit of a downdraft booth,
I just rear vented mine.
Good luck in your search and decision making.