My work table is birch cabinet grade plywood, 3/4" thick. Cabinet grade plywood is more expensive, but well worth it. Smooth top, not rough. 3/4" thickness would be thick enough for the span your wanting to achieve. I've been very happy with my work surface.
As far as measuring goes, have you thought about laser. A laser tape measure is very accurate, if you had two, you could create your 90 degree corners, and you could move them around.
Good luck with your design, post pics once completed to inspire us.😎