I gotta say, all of those aspects are important to me.
I'm trying to find one right now for my 80mm spindle with a 2.5" shop-vac hose fitting.
Most of what I have found on ebay are giant monstrosities you cannot see through and the ones that are clear, are way too tall and made to fit a specific CNC.
The brushes block the view at point of cutting and the height is not adjustable... I'm skeptical it would even fit between the spindle and the Z-axis mounting plate.
This might be fine for repetitious cutting where everything is at a standard height, but the vast majority of my jobs are 'one-offs' with many different length bits.
Sometimes I deep cut wood 50mm, sometimes I cut only 0.5mm traces into copper for circuit boards.

The spindle holder on mine leaves exactly 1.5" of the 80mm spindle available for clamping below the bracket.
The bracket has a thickness of .5" around the spindle.
The bottom of the collet holder is 1" below that... some of my cutting bits are only 10mm below the collet, others are 100mm.

So what I would love to see is:
A thin, strong clamp for the 80mm spindle roughly 1" of Z-height tall, no more than 1/4" thick to clear the inside Z-axis mounting plate.
A connector for the 2.5" hose on that with maybe a 5deg-10deg tilt away from the spindle to clear the holding bracket... otherwise it can be vertical if offset maybe 1" (hose plug-in is smaller dia than hose dia, you know what i mean if u have a shop vac hose).
Then for the bottom shoe....
We dont need a giant oval thing covering the entire area that the spindle plus hose connector takes up.... just need something like a PVC tube around the spindle collet that allows the same airflow.

Trick is to make the height adjustable, and to not restrict the airflow from the main 2.5"dia tube.
I'm thinking it would be best to have changeable shoe adapters, but each shoe needs to slide up/down a lil bit... even 5-10mm would do the job.

Actually now I am wondering if I can find an 80mm PVC Y-adapter that can slip over the spindle, I'll cut it down to meet the bottom of the collet, make an adapter for the 2.5" shop-vac hose... and just make a sliding sleeve to match the height of my current working bit. It unfortunately wont be transparent, but would be a better option than what I can currently find.

Let me know if you want to make something like this, I'd love to help with prototyping... Ill bet I'm not the only one interested.