A flat wing will rudder roll in knife edge. It just rolls less than one with dihedral.

You have 3 choices to fix the roll, mix, end plates or anhedral. I have flown a zillion Sticks and shoulder wing planes and have put anhedral in numerous. I think the manufacturers are afraid to put anhedral in a plane, thinking no one will buy it. Turn a 4*40 over and what do you have, a high wing plane with anhedral.

1. A lot of guys mix. You'll probably need 2, rudder to aileron for roll coupling and rudder to elevator for pitch coupling (most high wing planes move toward the top or need down elevator in knife edge).

2. You can saw the wing apart, put in 3 degrees each side, about 1 1/2 inches on each side, and glass it back together. I've done this a lot, too. It only hurts the first time you do it. After your plane flies better, you'll think it was all worth it. A radial arm saw works great. Set the blade at 3 degrees and zip. Set the other side and cut. I don't cut all the way through, just enough so I can bend the wing. I 5-minute the ends together, then use 30-minute epoxy and use 6 oz glass cloth 6 inches wide. Don't worry about a dihedral brace. I have glassed 80 inch wings like this and have yet in 35 years of RC to have a failure.

3. Add downward end plates. They should be flush on top, but extend down from the tip. You'll probably find that 3/4 inch down works fine, but make some from 1/8 lite ply so you can trim them. In one of my columns not long ago, I showed this technique. You fly and do knife edge or put in rudder from level flight. I started with the plates going 1 1/2 inches down on a Stick. With this much, the plane rolled opposite to the rudder. I landed and trimmed 1/4" off. I had lined off 1/4" lines on the plate. You fly and try, trimming until the plane holds pretty good knife edge. Once you find the correct depth, you can remove the lite ply plates and make some nice ones, covered to match the plane.