You don't need an engineer, just draw a triangle using the 3 dimensions you want the angle washer to look like. Then, use those dimensions to calculate the angle using the cosine divided by the tangent of the arc multiplied by the quadrilateral corollary of the late afternoon sun.
Actually, go to a search engine and type in 'triangle calculator' and it'll tell you the angle.
The hard part, of course, will be finding someone to produce said washers for you....cutting a washer on a slight angle, accurately, is tougher than it sounds.
If it was me...I'd forsake that fancy aluminum paint job in the interest of machining the proper angle into the intake boss so a normal bolt and washer would be used.