Filters on b/w film do not act directly on contrast. The change the way in which color relationships are rendered tonally.
If you have the filter ring diameter, you can look around for tiny filters. Here is a link to KEH's small filters: http://www.keh.com/Camera/format-Acc...c=79033&r=WG&f.
I would consider looking into one of the smaller Kodak Series filter systems if you cannot seem to find screw ins.