Well, after considering everyone's valued input, here's what I ended up doing.

Plan A:  I drilled straight down through the sight base near the back edge.  My intention was to put a set screw in the base that would tighten into a shallow hole drilled dead center in the top of the barrel.  My big mistake was choosing a 6-32 thread.  It's been my experience and the experience of a few machinists I've known that 6-32s can be difficult threads to tap.  My tap broke off flush just as it got through the hole.  I couldn't get it out no matter what so I just ground it off clean at both ends of the hole.

Plan B:  I drilled and tapped a shallow 6-48 threaded hole in the barrel at 6 o'clock.  I drilled a matching hole in the underside of the band on the sight base.  Once it's put together, a short set screw lock-tited into the hole will keep the sight base from sliding or twisting on the barrel.