Here is a link to a video I made showing a jig for holding your exact knife when cutting dollar tree foam board. It makes cutting control surfaces much easier.
I made 2 different ones, 1 for straight cuts & 1 for bevel cuts. The bevel cutter one is cut on a 60 degree angle & 35 degree angle. The 35 degree angle can be changed to whatever you want for the control hinge movement.