(Great minds think alike...)
I am on my fourth iteration for carrying Bay 1 filters for my Yashica TLR and the most recent is indeed the square filter case. B&H has/had a selection of sizes and they are inexpensive. I exercised an Exacto knife and carved up some scraps of craft foam to customize the interior.
The previous iteration was a $$$ two sided fancy tackle box which I cut through the divider in one corner so my metal lens hood would fit in. It accommodates four filters plus a two-part +1 c;loseup set plus the lens shade.
It nicely holds everything Bay 1 that I own, but partly because of the fancy rounded corners reducing the usable interior space, it is not useful for a simple pocket carry. Prior to that I had a couple of passes on smaller fishing fly containers which were at best OK, but didn't hold enough.
(Sorta like camera bags -- one winds up with a barn full trying to find a perfect solution.
)