When using a camera and lens combination which doesn't include a tripod ring; I really like an L-Bracket. The L-Bracket allows the photographer to shift from the vertical to the horiontal position while keeping the center of gravity of the camera/lens directly over the monopod shaft or tripod head.
Really Right Stuff produces, IMO, the neatest and easiest to use L-Bracket. However, this bracket requires an Arca Compatible Q.R. System.
http://reallyrightstuff.com/QR/05.html
I have an Arca System so I have no problems using the RRS L-Bracket. There is, however, an L-Bracket (of sorts - designated as the Junior L-Bracket 341) distributed by Bogen-Manfrotto for their RC-2 Q.R. System. I have never used this bracket so I cannot comment on its functionality. Although it doesn't seem as slick and neat as the RRS L-Bracket, it looks like it would allow you to keep the center of gravity over the monopod shaft or tripod head.
341 Bogen - Manfrotto Junior Elbow Bracket with 1/4"-20 Camera Mounting Screw, for any Tripod Head that Accepts the 3157N Quick Release Plate.