To add to this, when the dust boot is attached to the router, it will never be at the right height (just touching the workpiece), it will be above or it could slam into the workpiece if the bit reaches to the bottom of the workpiece. If it is made to touch when the work starts, it will squish the bristles when it reaches the bottom or even crash into the workpiece for thicker stock.
What @luc.onthego said.
It can still work if you are only doing very shallow cuts (e.g text V-carving) and there is very little Z variations in your toolpath, but sooner or later you will find yourself on those situations that Luc decribes.
If I were you, I would keep those 27$, and wait a few weeks until Carbide 3D releases their own dust shoe
I have a shield right behind my S3XXL and I need to remove the hose from the PwnCNC dust boot to allow the gantry to move all the way back for homing. If you design an enclosure with a bit more clearance in the rear, you should be able to make it work. I’m also planning to make a 90° elbow for the vacuum hose that should eliminate the need for any extra rear clearance.