Can I suggest building one? I built a full cabinet (about 350x350x1200mm) so that it's dust-tight; it's just transportable enough that I can carry it out to the shed when not in use. However you could easily build just the top box (air plenum with fan and filter) and drape a shower curtain from it. Some 5mm potplant chain would hang it up I guess; I'd expect the total size of the box to be about 350x350x200mm.
I used a 10" exhaust fan - they're cheap, mains powered and come with a plug so you don't do any wiring - mounted in the top face of the box. Mounted in the bottom face of the box just below the fan is a rectangular air-filter cartridge for some car; the biggest/cheapest flat rectangular filter I could find at the local car-parts shop. All you need is some plywood (I used 12mm because it's easy to put a screw in the end of it), a drill and a jigsaw, though having a table saw too helps.