Here is a reusable mold for a silicone boot for the Wanhao D6/ Monoprice Ultimate printers thanks to the original files by 1bigpig. It can easily be assembled and taken apart. Also increased the size of the cutouts for hot end and nozzle. Sanding improves fit and the look of the cast. I used Mold Max 60 from Smooth-on made for pewter casting which is stable up to 294C. Also use their Universal mold release. I mostly print PETG up to 240C. There is some H2O and ETOH evaporation (from the curing process) when you first heat these up, but no signs of degradation or smells since posting this albeit nonstop printing. This flexible enough so that you can easily get at the screws holding the heater and thermistor in the heat block.
Updated and fixed mistakes. Added an alternative mold for the bottom part of the heat block that better insulates and directs the cooling airflow to the tip of the nozzle.
I mix 30gr/0.9gr A/B components of Mold Max 60 and divide this to fill 3 molds.