[Phlogiston]

   

Yes - 2 halves.

See the little holes in the left half? They extend through the mold. You bend the leads from your replacement resistor (as large a wattage as will fit in the mold) so that they extend out of the mold.  The leads are left exposed as you remove the mold halves.

Yes. The easiest material to make the replacement from is vinyl - Bondo is a convenient source.

If you have the original resistor, removing the lead (metal) ends and replacing the ceramic center with glass (also a ceramic) is the best method if you think that the resistor will get (very) hot. The tiny resistive element in the originals was not capable of dealing with much heat - so your replacement probably does not need to (either).