When I first bought my #pinhole camera from #Ondu, I was amazed to find it came with a filter holder that attaches to the front of the case. This is in case you need to #attach an ND filter to slow the length of time the shutter is open for. Typically we need around 20 second or more to make sure there are no shakes when opening the shutter and to allow for manually opening and closing the #shutter.
The filter holder is designed for 'A' series filters manufactured by #Cokin, however these are only 67x67mm and are no longer manufactured (although if you're lucky you may come across some second hand ones). My only recourse was to get a #Formatt #Hitech filter which is 67x85mm in size and either modify the holder or the filter to fit the camera!