You can use a four way rasp/file for cutting or a band saw or jig saw.
For fiber glassing you need a 72 inch long strip of fiber glass and G flex epoxy resin. This is the only flexible resin I know of and it is necessary to use flexible epoxy.
For using cotton I recommend strong cotton material such as strips of Army camo clothing sewn together to make one long strip. If the cotton is more of a material similar to bet sheet material it will not do much good.