If you make it solid to keep the mould simple, then you can use our
Xencast P2 Fast Cast Polyurethane Casting Resin for ease of use and speed of cure. Add about 20% milled carbon fibre to it to add tensile strength, black pigmentation and reduced brittleness to the finished cast. If it doesn't turn nicely, you could add 5-10% graphite powder as that will help make the surface a little bit more slippery.
I would be tempted to run the mould split line horizontally across the step between the lid and thread of the cap. Silicone moulds should be flexible enough to still get the threaded bit out (or just unscrew it!!) and the flash line right on the cap tops base edge will trim off neatly and be almost invisible.