Up Date on Cones.
I just received a nine piece (9) white 'rubber' funnel adapter set from one of many sellers on eBay.com, and am very happy with it.
The largest cone has an opening fo about 8.cm and the smallest, about 9.5mm.
There is a solid piece to this set with a small face of 8mm +.
The wall thickness, on the largest is about 6mm, the smallest cone, about 2.8mm.
I have already used one of these to remove the front ring on a Nikon 50mm 1.8 'E' series lens that has never been dissembled, after leaving the factory, and it worked like a charm.
If anything, the only hick-up was telling how quickly the lens ring was loosened.
By the way, these are known as Buchner flask adapters for Lab work and prices vary with colour, seller, etc, but I spent about $13 u.s. AND I'M VERY HAPPY WITH WHAT I RECEIVED.
By the way, white and coloured rubbers are weaker than traditional black rubber, so if you find a black set, you may be better off getting that, rather then others colours, and buy the 9 piece sets, as they are durable enough in feel, IMO.
Lastly, I believe that if needed, a couple of boxes of Harbor Freight "O" ring assortments in SAE and Metric, can be used with these cones or simple High Density Foam, such as the shoe box sized foam blocks being sold for exercise.
Check out your local Goodwill and S.Army stores, etc, for used blocks you will cut down as part of your kit.