probably a small powered trim circular saw with a fence to make a straight cut.

Make the lines where necessary.

Use the jig saw to cut out most of the material, being careful to leave the lines.

Hopefully the two lower corners are rounded, since making 90 degree corners weakens the material.

Then, use a random orbit sander to sand to the lines. In the corners, use a round file, since no sanders can do corners.

I have rotary wood rasps---think a rasp mounted in a drill to do corners.

Also, a cheap jig saw is gonna cause more problems than it will convenience. Brands like Black and Decker/Ryobi/house brands saw crooked no matter how careful you are. The minimum I would use is a DeWalt---the best is a Bosch. Those start at $80 and $160 respectively.

Borrow or rent one if you only need it for this one job.

and use a downward cutting blade.