@RogerBill, cShells are very good, but I don’t know how are you sensitive to occlusion. Theoretically making large diameter vent may mitigate it, but that could again come with problem of underamplification of higher frequencies.
If you have problem with occlusion, ask your audi about deep impression (at least 5mm after 2nd bend of ear canal) so that cShell could reach bony part of ear canal, which can decrease occlusion when having small diameter of vent.
It could be easier for your audi when he would use EasyView Otoblock.
I have 1mm diameter vent in cShell.