I used a piece of Kingspan building foam that I had lying about. Approx 4" thick with a foil type covering. It worked really well and only took 5 mins to shape 2 pieces.
 
Ah kingspan... worked with alot of that when my dad was renovating his house... 10mm (4").
Use a fine-ish toothed tree/wood saw and it shapes easily enough.
If its crumbling, stop abusing it and be a bit more delicate :lol: :cartman:
 
Nope it didn't crumble at all. It was fairly new kingspan, old Kingspan when wet can crumble. I also covered the exposed foam with duct tape to protect it more.