Does it have to be a metal rod on the carrier, and/or get some brass rod equivalent from lowes, cut to length, file down the ends to a thinner diameter, solder em in
Measure twice, cut once, cuss once 
Thanks for that link on the 18500’s…
Behind that big flat slab of copper how much extra room is there for a driver, could you possibly use the potting method to fill in behind it for a bit more added thermal mass
silicon carbide and 30 min epoxy
I have a feeling this build is going to be SWEEEEET!