I just used Lee polyester (not resin) filters cut to size. You could use Wratten gelatin filters as well. I bought 100 mm square Lee filters, so that the remainder of the filter was still useful.

Here's the selection of 100 mm square (4"x4")from B&H.

It's a nice lens. Mine was stolen while being shipped four years ago, and I haven't replaced it yet.

Best,
Helen