Beautiful work, Megaton! Are you planning on building more?
If you feel constrained by the PT-20s, you may wish to consider mounting the galvo drivers (or at least one of them) to the underside of the top cover as it looks like you have "plentiful" available space there.The PT-40s that Eightonlight sells have drivers that measure 7cm x 7.5cm x 3cm and galvos that fit in 7cm x 6.5cm x 6cm or slightly less with cables like you are currently using.
-David