All square filter holders function about the same way: there is the holder and some sort of spring/tension plate holding the filter into the slot in the holder. I use a Formatt-Hitech holder which looks like this:
You can see how the plastic curves inward and it is flexible so that you can slide a filter in and tension holds it in place. This concept applies to most filter holders.
So, you can use any filter that fits the width of your holder as long as it also fits the depth of the retaining slots. In the image above, you can see that the slot is 2...maybe 3mm widest. I could use a filter that fits the width of the holder as long as it is no more than 3mm thick. Any thicker, and it may not fit into the holder - forcing this could damage your filter, holder, or both.
It is generally safest to go with the manufacturer's filters, but as long as the brand you are buying fits in length, height, and depth, there is no reason it wouldn't work.