When I did split filter printing on a Beseler 23CIIXL, I just placed the my unframed gelatin filters on the below the lens filter holder. But I did always want a set of those Ilford framed filters. Now I use a dial-in filtration Saunders VCCE enlarger, but rarely do split filter any more (except to dodge shadow areas with a no. 5 filter held in hand).

Jon