Sorry - I didn't make it very clear what kind of machine it is!
Here's a picture of the machine, it's a desktop engraving / modelling machine:
The table supplied with the machine is fabricated from steel and has a few sparsely placed Tee slots for holding down items. I wanted to make a tool plate similar to one of these, so I can fit stops, hold downs, etc:
The problem was that I didn't want to lose any of the travel on the Z-axis as it only has 30mm travel. I use the machine for cutting Brass, Plywood and Plastics. The sub-table could be Acrylic of even MDF if I was using it without coolant, but as I cut some brass on the machine I'd need something impermeable to cutting fluids. I could use Aluminium however I'd like to use something cheaper if possible!