I started this experiment this morning. I found that the cork ring I had here are only 1 1/4OD witch leaves very little room for the inlet for the knot. 1 1/2-1 3/4OD would give me more room. so I found one of the wife's old yoga blocks made from the same cork. Cork Rings are available in 1 1/2OD and cork blocks are also available 2"x2"x11" and both can be had in different colors and patterns. But for this experiment the Yoga block was just collecting dust and I can get 12 blanks 2"OD x 3" out of it.
Never having made a brush or replaced a knot I could use a little help on knot selection. I am thinking 23 MM max so I can have at least 8-9mm wall and may have to add some kind of thin walled ridged tubing inside to stiffen it up.