The beer keg k31 bolt handle does look interesting. I'd have to try one first to see how it feels, but I like the concept.
What I had in my head was something like the bolt handle/knob on the Browning Maral and Blaser straight pull rifles (except more or less straight out to avoid hitting a knuckle on the mag release). To be honest, it really only needs a ridge near the end for better grip, my soft city hands can just learn to toughen up.
On the way into work this morning I had the idea of using hockey tape to create a narrow, rounded ridge near the end of the factory knob. It's cheap, and easy to remove if I don't like it.