I made this slitter to cut a wide piece of foam weather stripping down to the size I needed.  It took about 5 minutes to make, and produced a very consistent width strip.

I started with a scrap of wood about 3" long and hot-glued a small strip for a fence 3/8" back from one edge.  This would be the width of the finished strip of foam.  The foam was 1/8" thick, so I made the fence approximately the same depth.  (The arrows indicate the fence)

I hot-glued a straight razor at an angle onto the side of the block, with the point of the razor extending a little deeper than the thickness of the foam.

I unrolled a length of foam tape on my work surface and ran the slitter down the length.  As you can see, it worked great!