I've used a roto-zipp and it was easy. I would be afraid that a jig saw would cause gel cracks from the excessive vibration. Use masking tape to cover the area where you will cut.
I forgot to mention that the area in carpeted and fiberglass behind...thanks i have the zipp and feel better using it but thought i would throw it out there.
Just remember to check where you are cutting. I was at the fiberglass repair shop yesterday and some bonehead cut into his transom while installing a ballast system!
I've used a Rotozip to enlarge a hole in the dash for a larger Steering column. It cuts through the glass like knife through butter. Works quite well.. I'd wear a mask.
it's a little easier to follow a line with a jigsaw than a RZ. i usually use a template or a rip fence if i'm concerned about the appearance or accuracy of the results with a RZ.