Looking for a radio-transparent epoxy/resin
You might not be able to find a home-usable thermosetting resins such as epoxy or RTV silicone rubber that meets your needs: low dielectric loss (dissipation factor), moderate hardness and stability at microwave cooking temperatures.

Bakelite (urea-formaldehyde thermosetting resin) might be usable, as it's hard and temperature resistant, though its dissipation factor is ~0.005, a tiny bit higher than ideal. However, one-off molding of Bakelite is non trivial, requiring pressurized and temperature controlled molds.

A table of electrical properties of some plastics shows polystyrene, polypropylene and some fluorinated plastics such as polytetrafluoroethylene with dissipation factors &lt;0.001, but these are less hard than Bakelite.

Glass-filled plastics such as glass-epoxy laminate (printed circuit board, PCB, material) and RO4000® hydrocarbon ceramic laminates might be better choices. However, instead of casting a part, you'd need to machine the part from one or more PCB's laminated together. This might actually be easier, for a one-off part, than casting.