Geoff, set up the piece of plate with a thick cardboard under it of the same size, place it over one of the Tee slots on the table and clamp it both ends, cut along the edge, the cutter can go in the T slot.
This will give you one straight edge to work from, deburr the cut edge and then rotate the metal and measure your width and cut again.
Again deburr the cut edge, rotate the metal and using a square set it up again over a T slot and repeat the cut, deburr and rotate 180 and measure length and finish the last cut.
Job done, I used this method on an engraving machine and it works fine.
To be extra careful, use three clamps, one each end and one at the middle of the rear edge.
This method will not cause any problem with the tool on the table surface.
Clive