I have tried the standard Ilford under the lens type filters but they interfere with the light path. The filter drawer in the lamphouse appears to take some kind of hard filter about 4 1/2" square but I can't find anything like that. I have heard of custom five inch under the lens type but that seems quite complex to make. My idea, which is more within my skill level would be to make some filter boards out of 1/4" plywood and staple or glue the Ilford 6x6 filters to the boards, cut down to the proper size, of course. Has anyone else tackled this?