As you're likely aware, cutting beyond the 45° limit of the saw can be challenging. It's necessary to think more vertically than usual.
Consider to build a fence that includes a 90° bracket along the entire length (or as needed to provide stiffness) that will permit you to place the angle as desired:
Blade represented in red, circular saw plate is yellow, vertical work placement allows for single light blue cleat, while horizontal work placement requires additional darker blue cleat.
Picture the circular saw running along the top of the image with the blade set to match the desired bevel and depth. If the bracket is the right width (narrow enough), it could be used as the guide fence for the circular saw by attaching an additional guide piece to the underside of the saw.
If the work piece is too large to be vertical and the guide bracket is cut narrow enough, the attachment to the circular saw could be L-shaped and hook over the back side of the bracket.