This is pretty easy just grab a jigsaw and cut the containers in half. There is a seam that you can follow all the way around and they cut easily with a wood saw blade.
Some containers have a deep recess in the bottom and can be difficult to cut all the way around with a jig saw. I cut as far as I can go in one direction then turn the container around and start again from the other direction. Using this method I can usually get the saw to cut through the recess.
Once in half you can wipe out the oil or wash it out. You can also sand the edges, but I don't bother.
If you choose to use a container that's had chemicals in it it would pay to wash it out before you start cutting it and wear gloves and goggles.