One trick to produce anhedral effect, which is what you want to correct roll in the direction of the rudder, is a downward onlt end plate. Make a couple of plates from lite ply or something similar that are flush at the top, but extend downward an inch or so. I would screw them on so they can be removed and trimmed.

Start with an inch or an inch and a half downward. The downward plate induces an anhedral effect, causing roll opposite to the rudder, countering the directional roll from the top wing. You'll have to "cut and try" to get the downward length correct. What you want at the start is a plate hanging down enough to cause opposite direction roll. Then you can trim a quarter inch and fly again. Once you get the size correct, you can paint or Monokote them to match and most people won't notice them.

I have used this trick on several planes to get good knife edge flight.

It's probably the sweep on the Ultimate that is doing you in. Sweep back, dihedral and a high wing location cause directional roll with rudder. Sweep forward, anhedral and low wing location cause roll opposite to the rudder. A designer has to "play" with these to get a neutral knife edge.