If the CG is quite a bit below the center of lift, there will be some pendulum effect that will provide you some of the righting you get with dihedral but I'm sure you will be far more satisfied if you add ailerons. A slight improvement can be had (if you do not add ailerons) is to cant the rudder hinge line aft at the upper end. I have seen some models of J3 Cubs with no dihedral and no ailerons fly reasonably well.