Coping Saw! or for even finer work- a Fret Saw!
Steve, that is a great Mod report.
My only concern would be 'Damp' entering the plywood on cold dewy nights and making the wood swell, hence putting everything under tension and forcing the mirror out of collimation.
A solution to the problem, if it occurs, is to use Perspex, Acrylic or Nylon Sheet for the plywood parts.
Well done, and instructions are written clearly.