Ok the vertical risers comment- I meant at the moment the side members that the long axis rails sit on would need to move up to push the gantry up and that it would be stiffer if the vertical parts of the lower frame were longer rather than finding a rectangular section which was very tall and slim. In fact I would probably not go with side members quite as rectangular as you have drawn them but make them a bit more squarish. Then get the height you need by making the base frame vertical parts longer. Very tall large open rectangular sections will start to lozenge when they get pushed sideways by the cutting forces. Away from pc so I can’t sketch something.