Hey Bob, hey Peter @Nightowl,
you are right, Riser blocks seem something easy to add but most people underestimate the increasing leverage force. Nonetheless they are offered.
But the the increased leverage force acts on the two X rails then via the Z gantry. I would avoid that too.
I plan a table where the room where usually the wasteboard takes place remains empty, with the possibility to mount different wasteboards “floating” when necessary and more room for larger workpieces or a large 4th Rotary Axis.
But when thinking of such things one must be aware how strong and stiff such a U-shaped base has to be. You will either have to use thick dampened beech timber to construct such a thing and use triangular reinforcements or, as I plan it, weld it from steel profiles.