The plug was obviously the finished machined aluminum part. The mold material is Synair Por-a-mold 555 urethane. I chose this 55 durometer material as a compromise, its stiff enough to hold dimensions pretty well & still has some flexibility to demold complex shapes. (It feels about like an eraser for reference). Synair is nice stuff for this type of application - equal mix ratio, rapid set up time, resilient to demolding. Just have to watch you don’t introduce bubbles during mixing, or vacuum them out or something. I used their recommended releasing agent. The layup was done with carbon tow & laminating grade epoxy resin. I don’t know the final weight off hand but recall it ended up about 70% of the aluminum part. That works out pretty close to what you calculate based on material densities. The carbon part ‘feel’ is incredibly strong & it fits the spinner cone perfectly.
Here are the links to the products
http://www.synair.com/art/grid.html
http://www.synair.com/products/releases/sl531.html
Here is a link showing some nice work that is similar.
http://winshiprc.tripod.com/index.htm