just get some baking paper and use it like tracing paper draw around the side you want to keep with a permanent marker.
This will then give you the same radius when you flip it over and the straight edge. then just tape in place make sure its the right length then using a sharp knife do a light cut to cut the fabric then a second to cut through the foam/rubber layer.
but thats just me, oh and for the straight edge put something like a ruler or something hard under it so it makes sure you have a truer edge.