Found this thread again. I've thought up another option:

I have a full set of 77mm filters I would like to utilize for my pinhole camera. In order to do this, I ordered a threaded step up ring which will be glued to the camera to mount these to. Now, the real question is, of course, behind or in front of the lens? Much easier to change on front, but the concern about extreme DOF is legit. Maybe behind and reaching in, who knows...