I have used the jigsaw and I have also used a dremel with a cutting wheel, it melts but nothing a sharp carton knife won't cure. The dremel gives straighter cuts.
Nothing works as good for me as one of these blades in a Olfa knife. The cut is so smooth and as straight as you want compared to a jigsaw blade. Takes sabout 4-5 passes to go right through a XP side panel. I've done quite a few of them with this and blades last forever.
Roto-Zip on one side and shop vac on the other to catch the plastic chips. If youre doing it on the sled you'll want to cover the intake and clutches cuz plastic will fly. Good luck. EW
Another option I did on my '08 was to drill a pattern after taping1/4" lined paper to panals. 3/16TH bit and line inside of panal w/screening (door/window, plastic/fiberglass). Glue in place or use Gorilla tape to hold edges of screen in place. Tape has advantage of easy change out if something tears or falls apart.
how i do it is i trace the vent on the panel with a tire marker. Then i take a drill and cut about an inch inside of where the vent mounts on. I drill holes in a strait line all the way around, and then grab a charp knife. its super easy!