I'd just use a V-block lying on its side against the mill vice's fixed jaw...
That may be Too Simple though, I'm kind of assuming you have a mill vice that'll open far enough!
If your vice isn't big enough it wouldn't be a hard job to make an impromptu (just-about-accurate, not talking surface ground!) shallow V-block that *will* fit in with the 'bar riser?
If you have a lathe with a 4-jaw chuck, you could centre-punch the hole location, get the punch mark centred with a wiggler and drill from the tailstock - would be my easiest solution if I hadn't the big-enough mill vice. Judging from the risers' appearance, you have a suitable lathe...
Dave H. (the other one)