I was going to suggest the Stax sign system which used to allow slide in substrate, but I see that Sign Conex now distribute that product so maybe the extrusion has changed now.
The signfix systems and spandex info panel systems used to have various grooves running along the inside length of the extrusions which I am certain would allow you to slide in a panel.
Sometimes we found it easier and cost effective to buy in standard off the shelf extrusions and screw/tape them together to create our own custom made directory panels.
http://www.asdplc.co.uk/
The ASD Metal website has a number of miscellaneous channels which may do the trick.
🙂