For any draft... Go with the direction of flow... Which is generally downwards for a plasma (only ever seen one vertical plasma table).
If you do go for a fan ... Look for something like a mixed flow fan and not just an axial fan as they have a higher discharge pressure for ductwork (flexible or ridgid)
(Dayjob tips)
I did sandblasting in the garage once upon a time... Best thing I ever did was build an enclosure using PVC drainage pipes with 1000 guage plastic sheeting and a 12" vent axis powerflow fan.... No dust ever escaped the enclosure (I used an air fed blasting helmet whilst sat in it for the time it takes to discharge a 50 litre pressure pot blaster full of 60/40 garnet... Vaccumed up using an old dyson for reuse as the cyclone separates out the fines.... Sorry off topic more to give an example of fan flow.
If you want to make an angled chute below the table for iron filings collection, need to consider the angle of response for iron filings... Which I think is about 45 degrees, hence I would split the underneath of the table in two and create 2 4' chutes, with an easy to empty iron filings tray at the bottom of each chute set as a drawer and a 12" extractor fan connected near the bottom of each chute, except you want the chutes to decrease in size to a sort of cone shape, then have an abrupt expansion where your drawer is, so that the air velocity drops off drastically and any suspended particles in the airsteam loose suspension and end up in the drawers.
Just another option to consider.
What I did with my slats is I got some 15mm SHS, and welded it to the end of some wide flatbar 50x3, which was cut in a chopsaw about 3" longer than my bed gap. On the table railsi welded a 50x50 angle iron at either end and then aload of 15x15 SHS teeth vertically which had a gap between them of about 20mm. Then I force and bend the slats in and they interlock, and they snake across the bed in a curved profile. They can be removed, flipped over, swapped or put closer together or further apart as nessesary.