Replacing the aluminium baseplate with mdf
I have now successfully replaced the (overly flexible) aluminium baseplate with an expendable (but rigid and perfectly flat) 16mm mdf panel 600x710mm giving me 40mm clearance on all sides.
This allows me to cover the emblaser with a box made from pine and mdf to keep dust and other stuff off it.
It also means I can easily drill as many holes ask like in order to secure my material. In this first iteration I have put another sacrificial sheet of 4mm ply on topic the mdf (screwed down) and then I used a bunch of mushroom head screws to hold down the edges of my 3mm ply that I am cutting.
While this does not provide any airflow underneath the material (am working on that) it does mean that the ply is held down perfectly flat, without the dive weights I was using before getting in the way. I have raised the height of the laser shroud up so that it clears the top of the screws (I'm actually using one of the screws to set the height of the laser module) and refocused the laser slightly to account for the additional height.
All looking good so far.