"Lifted" from Stephen Gandy's pages -"You can use filters, but only if they are much larger than the lens, such as gelatin filters and say, 72mm filters. Play a around with it, whatever works, works. You will have to tape it, or more likely, hold the filter in front of the lens during the exposure. Another option is using the 12's 77mm filter adapter. The 15's barrel is a bit smaller, wrapping some electricians tape around the lens barrel will allow you to mount the clamp on 12's 77mm filter ring."
Personally I've managed without using a filter with this lens and changed paper grade when printing (or fiddled with lighting and contrast if the neg has been scanned).