you may want to consider making the based slightly larger than the top. With the monitor up high, it will be top heavy and subject to tipping over.
I am interested in how your project turns out, I want to build another cab, but running out of room in my small basement. Have been trying to figure out how narrow I could make one with a vertical monitor, but still be stable.
luck,
edit: re stability, the diagonals may be what you want to look at with the design of your base.