Thanks Stuart, what troubles me with the MDF wasteboard is the give toward the middle of the machine, just a little pressure will move it many thou. in the z direction. Following a thread some time back in this forum I have removed the feet from the XL and fitted a rubber mat underneath. http://community.carbide3d.com/t/sounds-suppression-xxl-bed-flatness-and-other-musings/4875 This has improved the situation but still leaves a lot to be desired.
I agree that a sacrificial wasteboard the size of ones cutting area mounted on top of the ‘baseboard’ is a good idea and this is the way I intend moving. Flattening the ‘baseboard’ has problems by leaving a ridge that could get in the way with some jobs.
I think the bamboo will be fairly rigid and I probably will spend the money on the 25mm bamboo sheet, fortunately I have a friend who will take the other half sheet off my hands.