If you just have two positions - one low down for sheet material and one high up for whatever, then you can use blocks. Put a block on both sides to rest on for the low position and two blocks that make contact with the top to align it for the high position.

To get it level you can put a DTI in the spindle / z-axis, touching the bed and move the Y-axis. If the bed is not actually flat you'll still need to skim it but that should get it close.

I'm making the height of the bed on my machine adjustible with two jacking screws, and bolts into the 6 supports on the main frame.