The little tubs that come with the filters from Svbony are great but they have so much free space! This lets you put two filters in them.

If using with Svbony filters, place the insert on top, it'll fit on top of the foam and still close (that's how much free space there is!)

If you use different brands, such as celestron (which are taller) split the bottom foam piece into two (it's two bits glued to each other)