The roundels provided in "War of the Ring" and its "Expansion" are useless for stacking. They are too slick and slide off one another. Three-quarter inch stackable poker chips stack better.
Marshalling boxes on the side of the map would work too. E.g., marshalling boxes could be inscribed "Mordor North Army," "Mordor South Army," "Harad," etc. Inside the boxes would go units replaced on the map by chits of the same inscriptions.
All the longtime players dispense with the cardboard chits, and use a sort of base-to-base-contact rule and center-of-mass-rule to figure out the location of a huge glob of orcs. This avoids the annoying indirection of army boxes, and works fine even in the narrow regions like Ithilien.