Seeing as there are a lot of suggestions for tools you don’t have, may I suggest an Altendorf F45?
It’ll give you a fantastic cut.
If however you want to use your jigsaw, accept that it’ll potentially be a bit rubbish (I think you already have), use a straight edge to run the body of the machine against, cover your cut line in masking tape rubbing it down really well, use a nice and wide and brand new blade, and take your time.
Got a plane? Take off some of the inevitable blade wander with that, sand off sharp edges, seal ends with paint, job done.