Meanwhile, back at the ranch.....
Make yourself a pattern from 1/8" ply like so, making the outer and inner dia to those in the plans
Mark and then Cut your polystyrene outer circle - as best you can keeping the edges perpendicular
Next - IMPORTANT! - flip the circle over and line up the outer circle as best you can with the bottom circle you just cut and then mark the inner circle.
I use a 25mm ( 1" ) wide heavy duty box cutter - nothing less will do. IMPORTANT - I put the round template underneath the circle for support and to prevent undercutting the filler. I also have the rubbish bin underneath as polystyrene makes one heck of a mess.
Start by trimming a little off at a time, moving the circle around as you go.
IMPORTANT - here's how you hold the filler - not that the fleshy part of my thumb is the only part exposed the cutter as there are less tendons there than there are in your fingers!
As you keep turning the circle trimming a little at a time, you beging to get close to the shape IMPORTANT! - use slicing strokes rather than cutting strokes to prevent breaking the polystyrene away.
IMPORTANT - before I cut my slots, I made sure I marked the filler position with a compass
Differing from the instructions in the plans, I glue and nail the whole filler in place.
It's a lot easier to make a straight cut using the 1" blade fully extended by lining up the blade along the slot sides. IMPORTANT - make quite a few small depth cuts rather than one big deep one.
