You can use a "door handle" hook as an anti-sway device as in ortleib/carradice fittings.

Repurposed correx election signs are the best material for frame stiffener. The is one reason for Carradice Super C beng so lightweight for their toughness. Mine are over 15 years old and going strong.
I would recommend that the corners of Correx board are radiused to prevent stressing the bag fabric.