I used some scraps of Sky Shark P-100 with Avia .2400 glued inside, as I didn't have any solid carbon rod in stock.
It isn't the strongest material to use, but I have 4 or 5 sets of the older Rev handles and wanted to lengthen some of them. I don't power kite with them; I only use them in lighter winds, as long handles exert quite a bit of torque on your wrists and I found that I can fly longer without strain with the regular handles in stronger winds.
I make no claim that I've innovated anything; I'd heard of people using carbon tubing to extend their Rev handles and thought that I'd give it a try on my old handles.
It was so easy that I thought I'd share it and maybe help someone else.
Do be careful though, the stainless steel often has sharp burrs on it that are covered by the vinyl end caps.