I use a jigsaw with a wood cutting blade. Cuts through like butter or straight cuts. If making a tight curve or turn it will begin to melt the HDPE and gum up some because of the friction. Not too bad to deal with though. If only making straight cuts a circular saw would do too. Basically whatever you'd use to cut plywood will work just as well on HDPE.
Also if you don't have a jig saw, the belt sander will work wonders on corners of HDPE. Hope that helps! Although fiberglass is worth the time after you get her dialed in.