I'd think putting a filter between the negative and the lens is inviting distorted or otherwise degraded images. Far better to use the larger filters above the condenser. For this, I'd buy a set of Ilford or Kodak filters -- Ilford for Ilford paper, certainly. They're about 6 inches square and can be easily cut to fit the Beseler filter drawer. I wasn't aware that the 23C ever had a drawer between lens and negative. There is a slider that moves in and out from the lefthand side of the lens assembly just above the lensboard. In my 23C it held a thick red filter, which with some graded papers permits turning on the enlarger with the paper on the easel. I think this slider was not intended for use with VC filters.