My goal in building a multi-material hotend is to try merge the materials as late in the hotend as possible. In the Prusa MM design, the materials are merged just before ending the heatsink at the top of the hotend. Instead, I want to merge the materials just as they enter the header block.

Why? Partially it's speed so I can switch material which less retraction. Partially its reliability since I've found the large retractions used in the Prusa design often leads to the filament blocking. And partially is to design a multi-material system better able to handle flexible and other "difficult" filaments.

For test purposes I'm using a E3D V6 style heater block (a knock-off for sure) as, ideally, I dont want to redesign and rebuild every component of the hotend. Normally a heatbreak and heatsink screw into this, and so it's these pieces I need to replace with my multi-material part. The trouble is that whatever part I use here, it has to survive being heated to at least PLA temperatures (and ideally higher) so I can hardly print it using the Prusa and I dont have the machinery to CNC a metal part.

But I do own a Formlabs Form2 printer, and Formlabs produce a High-Temp resin. This resin is designed for injection molding and has a high heat deflection temperature even under large pressure. It might just work.