Here is the chassis tub all 500mm of it !
The finished model will be 612mm long in the end.
Now that it has arrived I can see I have 97mm between the inside walls which lends itself to the fitment of a Heng Long Dual Current Driving System CNC Gearbox I reckon as that is 95mm wide I believe.
Will have to have a chat with Forgebear about availability of the longer 65mm long shaft version as I suspect that will be the version I will need.
Also have ordered myself a larger A1 sized cutting mat...

There are many paths to the top of the mountain, but once there the view is pretty much the same.