Tom,
Take an older, worn out cork grip on a rod you never use anymore (got any kids that never use a cheapo rod?) and sand the cork grip to obtain a small pile of cork "dust". Mix this with a glue of choice (Titebond 3 or??) and use this "paste" to buildup as you need. Depends on how large your voids are but you can build this up over a couple days, allowing drying between adding layers.

I'm no rodmaker, just a woodworker but this is what I'd do.

Jeremy.