Anti-rotation washers (non-turn) washers in non-parallel dropouts?
The torque that the axle sees relative to the frame can happen in both directions as gears change.

For example in the classic Sturmey Archer 3 speed, it's the second gear that is neutral and does not place a torque on the axle.  In third gear the axle will try to twist forwards, and in first gear it will try to twist backwards.

For this reason you really do need an anti-torque washer and you can't leave the gap in the second picture above or below.

I would try to use a normal anti-torque washer against the inside face of the dropout.  This washer will have the square hole to grip the axle.  Then I would try to make a second washer-wedge-combo out of a softer metal (e.g. Al) to go on the outside.  The wedge part of this piece would fill that gap between the red lines in the second picture, but it's strictly functioning as a wedge and can have a round center hole.