As I mentioned in a previous post. I tend not to carry a square filter set with me except when I am specifically going out to shoot landscapes. I used to carry the square filter setup but realized that I seldom used them, I didn't want to take the lens hood off my lens and set up the lens to take square filters and just holding the filter in front of the lens without using the adapter would often cause a jiggle, spoiling the sharpness.
However, I carry my 77mm round filters using stack caps and carrying an extra filter or two adds almost no weight or bulk to my gear. The bulk is as important as the weight since I frequently will not carry a camera bag but, rather carry my gear in my photo vest.
I decided that I wanted to have a ten-stop ND flter along on my shoots and looked into purchasing a round 77mm. Many of my lenses have 77mm filter threads and I have a 67-77mm adapter to use the 77mm filter on my EF 70-200mm f/4L IS and EF 35mm f/2.0 IS lenses.
This seems to be a very decent review of the popular round ten-stop filters.
https://www.the-digital-picture.com/...ty-Filter.aspx
Until I read the review, I was leaning towards the Hoya PROND brand (which had some very nice comments on YouTube videos) but, decided to opt for the B+W. Since I shoot in RAW, I can adjust the color balance quite easily but, apparently the Hoya degrades the image sharpness. The Singh Ray, Heliopan and Breakthrough are great but, cost more than I want to pay (considering the few times I will probably use the filter). Given, the price-point and quality; the B+W seems to be a decent compromise...
BTW: I was able to find a pre-owned B+W 10-stop filter at quite a decent price.