Hello, Steve:
One respected builder of elk risers told me that he puts a "rod" in his, but the riser is amply strong without it. I had two separate conversations about a month apart about limbs. In the first he indicated he preferred bamboo and glass over foam and carbon and in the second conversation said either bamboo or foam with glass, but not carbon. The lead time was almost a year so I passed.
If serious, might want to be very clear about what works and doesn't work for limbs.
Regards, Kevin