Thanks Irving, very helpful!
I was thinking about using 20mm rail so that sounds like it will be the size!
I am thinking about mounting the profile rail onto a 40mm Extrusion and use the T Nuts to hold it in place.
It should be pretty straight bolted to the extrusion
These two extrusions will bolt onto an aluminium plate either side 10mm or 12m thick, not sure yet....

Andy