I like the concept of the Dust Cap, but I don't like the 1/2" diameter elbow, which seems quite restrictive. I'd like to open that up. Also, the Dust Cap is is a fairly complex piece to build. I've had some time to work on this and played with a few ideas, and I may have come up with some ways to simplify construction and increase the airflow. This is what I am working on now.
I found that there is enough room to run a plenum on the right side of the chuck. I've made a prototype base piece with the plenum opening in place. It fits. I am using Baltic birch at this early stage, but will probably switch to cast acrylic after I get some. I'm using an aluminum base plate, at least for now, because it was easy for me to fabricate. A wooden plenum mock up proves there is space. The actual plenum may be made of glued plexiglass sheets or bent aluminum. The rectangular shape eases construction. I intend to make a manifold for the top of the plenum to exit through the slot opening in the machine's cover. The manifold and/or plenum will be attached to the top of the y-truck. There is almost enough room in the slot to fit a 1" pipe or hose without modifying the cover. If the hose is mounted in a U to the left, there is no need to account for hose rotation.
There's no room for a dust cap top cover with my CarveTight, but it might be possible to glue a very thin and light plastic flange to the chuck, to expand the diameter of the chuck to the ID of the dust cap, acting like a piston most of the time if chips flying out of the top are a problem.
I may try getting by with just an open cut-out for the bit plate. With the plenum right over that area, it may work. But if not I think that a sliding ring on a pivot might work. I made a cardboard prototype to test the concept of having the bit plate push it out of the way. My screw locations will need to change a bit for this to work, but I believe that once I do that the ring will fit without modifying the machine. I may cut out a ring of this type to test if it will actually work with the machine operating as my next step.
I thought I'd post this as a work in progress to get feedback.