First, DO NOT heat fuse kevlar. Bad juju.
Second, cutting kevlar is a nightmare, by design. I use a fair amount of it (non-ballistic) for aircraft and motorcycle parts and have a special pair of ceramic shears specifically for cutting kevlar. It's possible to cut it with a razor blade, but make sure it's brand new and you have to push down HARD to make a clean cut. Never try and go over it in multiple passes. It'll just turn into a big, yellow fuzzball. I think what I'd do is stitch a loose seam around the outside edge then cut to about 1/4" outside that with ceramic shears.
I need a frame sheet for my pack, too, but I'm going to do it the cheap/lazy way and lay up a 5 layer kevlar (again, non-ballistic) sheet with vinylester resin on a foam mold. The vinylester resin will let it flex without cracking and I can use unidirectional cloth to tune the flexibility to the directions it needs, where it needs it.