Hi,
I have always used the 4mm Correx too. I think the 2mm will be too flimsy. I have used smaller pieces of correx and taped them with duck tape to make a big enough liner. I also use a knife (Stanley knife) with just a few mm of the blade exposed and score the correx using a metal ruler. I cut an extra "tab" for the corners, which i just bent round and tape in place.
Helen.