This solves the problem where the paper filter collapses on the pour spout and causes the coffee to be over extracted.

You need a standard stainless steel straw, (~8mm Diameter) I bought this one as a 4 pack at walmart
I printed this in PETG because it is mostly stable in heat, but the coffee does not come in contact with the plastic anyway, so I am not all that concerned.

Print it, stick the straw in, place the straw in the Chemex pour spout. One the brew is finished, remove it with filter and grounds before you pour your coffee.