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A simple job of removing material with a bobbin sander in your battery drill. The panel appears too small until you have marked and cut out slots for the side bars on the chassis. Then you can mark around the rest and trim accordingly.
Shown is the standard PVC board but the same applies when you are fitting the GRP G-type side panels.
2 Standard panel sizes are 303x1824 and the shorter one is for your rear bulkhead. You can use the off cut to create a triangle to block the IVA inspectors ball from contacting sharp edges on the side of the pedal assembly.
You can also mount side repeaters in these triangles if you do not have G-type wings with side repeater mounting points.