Solid Matte Box Support, but Poor Spacing for Dovetail Plates
The support is exactly what it says - this screws in to the base of the Mirage Matte Box and has a height-adjustable bracket to connect to a pair of 15mm rods below. It's well-made and solid.
The problem is that if you are using a Tilta 15mm dovetail rail with your rig, the bottom of the support bracket does not clear the safety screw on the front of the dovetail, which means you either have to remove the safety screw or you can't move the rig forward on the dovetail to balance it properly. I much preferred the single-side support from the Mini Matte Box, because it doesn't interfere with the safety screw on the dovetail.
If you aren't using the LWS base/dovetail system, then this will not be an issue.