If you have none of those, PM flakko. i think he uses a steak knife or something.
When drilling the hole for the jigsaw, drill it inside the ciricle and then cut outwards to get to the line. Or if you know what you are doing, you can also aim the bit so the outside of the bit is tangent to the circle.
If you need to buy a jig saw, I reccomend one that has the capability to select a straightness. Also use the narrowest blade available as that will reduce stress on the jig saw as well as the blade.
Good luck!