A folding 3-bladed prop might help. Then again, the rudder was designed to work with the boat going forward, so could increasing reverse thrust simply make it worse, with the rudder stalling out sooner, because you're going faster and losing contact flow before you used to? Lots of guys get into plenty of trouble in reverse, even when they have good steering control; your lack of control may not be as bad as you think. If you can experiment without actually buying the prop, it mght be worthwhile. Dave Gerr has an interesting book - "The Nature of Boats" - that goes into some of the issues involved with propellers and the surprisingly big results - both positive and negative - that apparently minuscule changes can have.