long exposure – Ho do I take slow pictures when the camera is too bright?

I recently bought a Panasonic FZ300. The camera is good, but to my surprise I found out that minimum aperture is 8 and minimum ISO is 100. I tried to take some pictures of a stream with the circular polariser mounted, but still I could not get the running water effect. I took also a couple of over exposed pictures and tried to correct later the exposure with Darktable, but the result wasn’t very good.

I heard I could use a ND filter, but beside the fact the with all accessories I bought I’m already over budget often the scene is always changing, animals move, the clouds that make a curious shape drift away, mounting a filter takes time.
Are there other solutions?