Looks good, but you could maybe improve a couple areas:
- Cardboard can lead to dust in the air and paint, especially those trimmed edges, so you might want to build one of out plastic at some point. I'm moving soon and my plan is to make a skeleton frame out of wood or metal, and make the walls out of sheets of clear polycarbonate to let light in.
- Drape some plastic sheet over the opening to keep dust from settling in the box when it's not in use.
- I don't know about the exact risks, but when you have an electric motor in the air stream, you do risk a fire/explosion unless it's an explosion proof fan.
- Testors sells the filter sheets for their spray booth separately. From what I have seen they use more than just the sheets for filtering, but those should be better than anything made from paper (Which would also contribute to any dust in the box).
- You could try some adhesive-backed foam wrapped around the hose coupling to seal it up better, if you want to remove it (Wrap the foam around the coupling and squeeze the hose around it). If it will be in place for a while, regular silicone sealant will work.