This is my initial design:

Two 4mm tubes enter the heatsink at the top, converging at the base. Inside these tubes are inserted 4mm OD/2mm ID PTFE tubing which carry the filament for the majority of the heatsink. Towards the base, the heatsink tubes shrink to 2mm tubes to take just the filament to the base.

In fact, these two 2mm tubes merge not at the base of the heatsink, but at the base of a screw thread which will be inserted into the heater block; the idea being that only as they merge will the filament be melted; at the last possible moment.

The heatsink is sized for a 40mm fan which will keep the heatsink cool.

There are quite a few unknowns which this design. The two major ones are:

1. How thermally conductive is the high-temp resin? This could effect not only how well the filament melts at the merge point, how well the fan keeps the heatsink cool, and what kind of heat-creep it might experience.
2. How strong is this material? As the filament is pushed into the heatsink, will the screw thread break as, at that point, the material is not very thick.