Ok, since nobody has mentioned it: a router template. Let's say you want to do a christmas tree shape out of 10mm coro.
20 of them
Get a chunk of MDF, jigsaw to shape, cleanup the shape by sanding etc.
You could do only half the tree if you want to save 'time'....just flip it ( mirror)
Clamp the coro to the mdf, stick a router bit with the little wheel so it just clears the coro,
and follow the shape with a basic router or one-hand trimmer.
Sometimes running a cleanup pass in the opposite direction gets rid of most of the little 'feathers' where the flutes don't cut cleanly....or trim with a knife.
4mm you can do a small stack at once.....speed and direction is important if you don't want them to stick together ( melted edges).
Here's the actual 'irregular' shape that I did a batch of ...from 10mm coro.