POLYGON can't help here, I'm afraid.

An entity with their POLYGON flag will be a per-polygon obstacle to other objects using c_move, but when it uses c_move it'll use an ellipsoid hull.

GS's collision detection options are extremely limited, unfortunately, and previous requests for improvements in this area were deemed too much work. You're better off somehow taking advantage of PhysX, or a collision-detection plugin, or writing your own collision detection, or settling for ellipsoids.