I have the regular Cokin resin filters for Cokin P. Even with "just" 3-4 stops (for example, one nd and one graduated), I sometimes have to watch for light leaks, and they're less obvious than what you posted.
The first photo you showed is typical: sun just out of the frame, some stray light enters at an angle above the filter. Instead of buying more filters, you could try to seal the space between the lens and the filter. It's simpler to use a single system than to manage step down rings between different lenses in addition to the square filter adapter.
Here's a tutorial that covers multiple aspects of light leaks, including square filters
Long Exposure Tutorial: Eliminating Light Leaks | Mabry Campbell Photo Blog
(btw, I like my screw in 10 stop haida, I got it in 49mm for da limited and it was affordable)