100 mm square filters in a ProShade 6093/6093T, or 75 mm gel filters in any of the older ProShades.
The older proshades will not let you combine such filters with a glass filter on the lens. The 6093/6093T will.

Glass filters will vignet when you put more than one on the lens itself.

A bayonet 60 to bayonet 70 filter adapter (as is used on the CFi FLE version) should work too.