QUOTE How do I get a difference in height of 15cm with a length of 40cm with 12 cm MDF?
Frank, tell me if I understand...
You want to create a difference of 150mm in height, with a piece of MDF 400mm long and 12mm thick?
The MDF will bend, although you may need to do it gradually.
I have read of people using water, but I would think this would be no good for the MDF after a while.
Try increasing the height a bit at a time, maybe a cm or two each day... let it take shape slowly.
Or cut the underside with a saw or router across the board, this will allow some flex too.
I hope you understand...