It would have been so easy to combine this long exposure with a fast exposure into one image, but I am glad you didn't. I think the image is far better
with the motion captured and retained within it. An excellent image!
I suppose everyone knows that you can do the big stopper thing for as little as £1.20 don't you? -
Go here - I think using these pieces of 10 stop glass gives you a slight colour cast on your image, but it is such a cheap option, and you can soon filter out any tints in post and of course that isn't a problem at all if you convert to mono

Dave