Where can I find the rod clamp that sits under the Matte Box and attaches to the dual rods as it is shown in the product images?

The lens support shown in the image is discontinued. However, we have other lens support. Please find below links for lens support and 15mm rods. https://www.smallrig.com/smallrig-15mm-lws-universal-lens-support-bsl2680.html https://www.smallrig.com/15mm-Carbon-Fiber-Rods-Pair.html?sku=5212

Can you use more than 2 filters trays? I would like to use ND, grad ND, and a mist filter at same time. If so can you buy the filter trays separately? Mini Matte Box Pro.

Yes, this matte box pro is supported to use more than 2 filters trays. For the filter tray, please contact us at smallrig@smallrig.com to buy it. It is not list separately for sale. Thank you.

Does this system allow for me to use a screw-in filter on the lens? And, how can I use a polariser if I use this system - is there a rotating tray sold seperately?

1. Circular threaded filters can be used and are installed onto the internal threads of the lens adapter ring. Threaded circular polarizing filters can be mounted following the method described above, but the maximum outer diameter of the filter must be less than Φ92.5mm; otherwise, installation will not be possible. 2. Sorry that there is no circular filter frames are available.