I've tried everything, and the best tool by far, in fact the only useful hand tool I've found, is a long discontinued Eclipse sheet metal saw, they come up occasionally on eBay, and are worth searching for. Use a 24tpi hacksaw blade, silicon steel works best on fibreglass. An ordinary hacksaw is no good, as the frame won't pass along the cut. A coping saw is OK, but it's a bit difficult to cut a straight line, so cut a safe difference away from the line, and finish off with a sanding block. A Dremel? Not unless you're uncommonly patient and have vast numbers of replacement blades. Using a Dremel type tool on fibreglass is alright for small areas, very useful in fact, I wouldn't be without one, but you it's not a suitable tool unless you only want to cut two or three inches. Mark the cutting line accurately with masking tape, and only ever cut from the gelcoat side, otherwise you'll make lots of chips along the edge.