[Gromit]

I'll add a couple more tips and photos - It is actually better to make two of each of the hardboard patterns, so you can sandwich a piece of foam between them and use the wood to guide the blade along the profile. Even doing this (as you will notice in the attached photo) the foam wants to squirm around and the more it does the more sanding you'll have.
I tried putting short screws in the pattern to hold the foam better and that helped.
Out of the razor knife, electric dual blade carving knife, and regular butcher knife, the butcher knife worked best; but I had to tune up the edge after each cut or it would start grabbing the foam. I used the type of sharpener next to the knife in the photo.
My original factory foam had burlap glued (actually formed into) the back; so I glued burlap on with the Super 77.
I used a Surform with a 40 grit metal blade - I didn't try an electric sander -
Finally you'll need to vacuum all the sanding dust out of the foam afterwards; and I'd advise wearing a mask when sanding.