Follow up:
I finally got over to town and picked up some 3/8" Oak dowels. They work really nice. They've supported every rifle setup I've tried in it, even my heaviest varmint rifle and the 3/8" dowels didn't flex any. The fit is nice in the rubber and the piece designed to slip on and off the bottom works much better with the 3/8" than the 7/16" I tried initially, probably because that's what they were designed for. Anyway, very nice, solid product here, and the setup does make for a really handy, lightweight set of sticks.