If it were me I'd drill out the balsa above the tube at the stern (that's already a tail wheel bracket) and put in a simple caster wheel held in place with a set-screw wheel collar inside the fuselage and another one below the tube to act as a load collar (bend the rod to absorb some of shock of landings). That is if there is enough space below the elevator linkage. I don't think you have the clearance to rig a steerable tailwheel rigidly connected to the rudder.

Unless you run the elevator linkage off to one side or even outside the fuselage near the tail. No law against that.