Hacksaw blades make an excellent cutting tool. The serrated teeth go through foam nice and predictably. The blade is just the right amount of flex and stiffness to finesse the shape you are after.
Make your own handle by wrapping one end with duct tape or whatever else is handy. Or just grab it o' natural if you are manly enough.
It's not always worth risking the domestic harmony by swiping a cutting implement from out of the kitchen. At just a few bucks it is also not a heartbreaker when you forget in your paddling buddies car. Go for it.
Also check out our Prison Shank Kit which consists of this blade and a small roll of duct tape. Never again can they say you never made anything on your own. You will also be that much closer to being prepared for the zombie apocalypse.
Hot tip: serves double duty as a shaving tool. Scrape the blade sideways across foam to shave it down.