I’m a bit confused by the “negative pressure” suggestion. Given that the dust boot is already inside the enclosure, no further porting should be necessary to achieve negative pressure.
I’d actually have the opposite concern. Your enclosure looks like it might be pretty tight, air-ingress wise. But if air can’t get into the enclosure, your DC can’t suck it out at the boot, and your dust-collection efficiency will suffer. In the extreme case of a completely sealed enclosure, you’d just pull a vacuum, and get no airflow at all into the dust boot.
I’d consider adding some air inlets along the top and the sides of the enclosure. Ideally, you want the clean air coming into the enclosure through those inlets to pass over the ball screws on it’s way to the dust boot. That clean-air breeze should keep the ball screws as dust-free as possible.