to cut an accurate 1 foot square hole in 18mm MDF? My jig saw isn't the best in the world, so I'm thinking of other options. Sadly no plunge saw, but plenty of routers.
I would knock up a quick frame (think picture frame) with a 300mm opening. Cut out the waste with a Jig saw and then using a bearing guided router cutter clean up the edges. I'd then square up the corners with a chisel.
It's slighty long winded but gives guaranteed results. I used to do a lot of fire doors with various shape cut outs using the same technique.
Do you have a top and bottom guided bit? You can cut in with the top guided one as far as you can go then flip over an do the rest with bottom guided bit running on the first part.