The Crazy Max has ailerons. And a fully symmetrical wing.
The SlowStick is rudder/ elevator only and has undercambered wings.
A plane without ailerons will not turn effectively without dihedral. And a plane with ailerons won't turn correctly with dihedral.
(edited to add: not for all planes, there are exceptions such as low wing planes, a lot of which have dihedral and ailerons.)
Also ailerons do not work very well with undercambered wings.
You best bet is to cut the foam piece you installed out and put the dihedral back in, or cut the wings out near the tips and add some polyhedral. Which just means two bends in the wing.