Hi smilee,
I got some of these cubes off this guy, because he is close to where i live. i haven't put anything less than 23L into them, but never been a problem.
When i fill with hot wort i screw the lid on tight back off half a turn than put a towel around the cube put my knee on one side and my hands on the other and push the centre in till the wort is close to the top, than do the lid up all done. they come back to shape when empty.
it works out good when it is time to tip into your fermenter you get the cube on a bit of an angle before the wort starts running out, nothing worse than trying to tip a full cube and it starts running out just as you tilt it a bit.
you know these cubes have had vinegar in them so they need a bit of a clean but i haven't had any bad taste out of them.
Mal