One of issues I had this summer, whilst touring the UK in the camper, was a lack of ND filters. I chose to shoot film alongside digital, but this meant I didn't have any ND filters for the Canon M6mk2.
Problem solved - I actually found that three small polarisers stacked together can create the same effect as a 10 stop neutral density filter. The image of these guide poles shows just how effective that can be, giving me a shutter speed of 30 seconds and more, simply by sticking one onto the other with a bit of Sellotape! You have to rotate them so very little light gets through, but the result was superb all things considered.