Look at the Pictures in the bottom of this link.
I used a coin with a drilled hole in the center and a rod to press "groves" on the inner side where the sheet is bent.
Always reinforce the outher Surface with transparent packing tape before bending to avoid the depron to break up.
Some tape along the outside of the bend with the score or creases on the inside of the bend should help keep it from snapping. Also, make the bend slow so the foam has time to compress, like bend a little, release, bend a little more, release. Repeat until you get the full bend.