With original feet, an A500 is 51mm tall and 366mm wide. The depth from the back to the "ridge" is 150mm. (See "overhang" in the photo).
Monitor width (and depth) varies. For the depth, you might want the spacing of the feet rather than the full depth, since the tube can protrude at the back without support from the surface the monitor stands on.
With these measurements, you can make an upside-down "U" in any material, from which a 150x51 (plus 2mm margin) is cut out on the floppy side.
This photo looks like an expansion, where part of the plastic case of the A500 must be cut off to fit it.