With the baseball bat idea there would probably be a 1/2" to an 1" lip around the rod, which wouldn't be very pretty, but it wouldn't bother me too much (you could use a lathe to make a clean taper which would make it look way better). I would consider keeping the knob, it might give it a cool look.
But, I've pretty much dismissed that idea...for now. I'm most likely going to use an old graphite shaft golf club (as someone suggested above). I know from one random sport to the other, but bear with me.
Here's the new plan: it currently has a 10" handle I want it somewhere around 18". It's a split grip so I'm going to cut the rear grip off leaving approximately 6-7", ream out the rod to the proper fitting if it's needed (I'll have to dry fit it a few times to get it just right). In the mean time I'd cut the driver to the proper length, compensating for the 3-4" that will be fitted into the end of the rod. Then paint the golf shaft, put a new Golf Pride grip on it then epoxy/clear coat it. I also might stick a trim ring where the connection is, if I can salvage the one that's on the rear grip.