Cool, I have the rails, which I use with a handle grip esp. on C300 and F55, and use my Tilta rig with that, has a Fotga matte box. Also have the rails mount on my HDX900. Don't use filters too much, but sometimes for older people and polarizers if outside all day.
I use one without any filters when I want to look like a serious DP. I am being sarcastic but I've seen some posers who are most likely using one for this very reason. I saw a guy who is likely a sit-down DP that was hired to do run-and-gun ENG work. He was struggling with the size of his rig hand-held smacking his empty matte box on chairs, tables, columns, and people.
My matte box is used when I need the additional hood to control glare. This is necessary when using my Letus lens adaptor since the prime lenses don't have shades. As for filters I've used polarizers and Pro Mist filters. Have yet to use grads.