You can do a little, but not a lot. You can move the rudder aerodynamic center downward to be in line with the plane's CG (or as much as you can); do this by moving the rudder as-is downward or fattening up the portion of the rudder below the plane's CG. This will address the direct rolling action of the rudder, but what you cannot account for is the indirect rolling you will get- with the plane yawed in one direction (as when you are in a knife-edge), the wing that is more forward will produce more lift than the lagging wing (which is somewhat hidden from the wind by the fuselage), and your plane will roll in the same direction as yaw.

Moving the stab will not do anything.

If you have a computer radio, you might create a switchable mix of a little opposite aileron with rudder (which you can activate when you want to knife-edge, for instance).

No matter what you do, the plane cannot be set up to have no roll coupling in all regions of the flight envelope (e.g., fast and slow, heavy and light, etc.), so you will have to practice using that aileron to keep the plane still when yawing, whether knife-edging, doing a hammerhead, crabbing a landing, or doing a rudder turn.

Note that some planes will have more of an effect than others, and I believe the primary variables are the position of the wing relative to the fuse, fuse size, and the aspect ratio of the wing.