Without adding a trim tab of some sort, I believe you may be limited to changing the weight.
What about adding some small weight maybe inside the spoiler box on the left wing. Try a little bit, then add to it as neccesary.
Or if you have water bags, add some more water to the left bag, or a weight inside there.
My only other thought it to have your ailerons adjusted if possible. Such as the rigging of the pushrods to the ailerons, have them adjusted in length so in a neutral position you have more right aileron down than previously.
These are just ideas, I'm no mechanic. Hope it helps or spawns a new idea.
