Is it possible to get hold of some Cokin-P sized rigid optical sheets in 'very yellow' (I suppose 200cc?) and 'very magenta', probably from a movie supplies place? Long ago I was a customer of SRB in the UK, but there are certainly more options in the US. There is nothing magic about the manufacturer's filter material after all. These higher quality filters would be flatter, easier to clean, optically much better and be more robust in use than the gel multigrade filters, and should work fine for split-contrast printing. You could even buy a third one which might look like Grade-3, or so, to simplify the 'average' print.