I have obtained a used Redrock Micro MatteBox primarily for use with my XF300, and would appreciate thoughts on a few issues.

The side mounting means that the 15mm side bars come very close to obstructing access to the XLR sockets. I am fairly sure I can overcome this by using shorter side rods. The supplied items are actually a shade on the loose side, and measure 14.8mm. .2mm (just under 8 thousands of an inch) is one heck of a clearance in this sort of application, and they are difficult to clamp.

I can cut down the supplied rods, but I would lose the internal thread used by the end-caps. That appears to be 1/2 inch UNC, so easy to get a tap, and probably cheaper than new rods.

The donuts have very little to mount to on the front of the camera. I'm guessing a 82mm filter would add enough depth to fix this?

The donut is going to obscure the 'Instant AF Sensor'. Now I am sure some of you will say that in a situation that requires the use of a MatteBox, you would not be relying on any sort of AF, but there times when it can be a life saver. I am not entirely sure exactly how crucial the Instant AF Sensor is in this respect, but I believe it at least has impact on the speed of the camera's AF.

Any thoughts appreciated.

I use a TLS Falcon matte box that also masks the AF sensor to the side of the lens. When using the matte box, I turn off the Instant AF & rely on Normal AF. The sensor on the side is only for Instant AF, Normal AF is through the lens, so your matte box will not interfere with it.
Although I rarely use auto focus, I find the Normal AF works well.
Cant help with your bars problem as I use a Zacuto set up...
Best,
Chris

Thanks Chris. I contacted customer support at Redrock just after starting this thread, and have just had a reply asking for a picture of my setup, but I think they'll say use short side rails. How do you get on with locating the donut on the front of the camera? I've ordered an 82mm filter to give mine something to sit on.

One of the reasons I got the mattebox was to be able to use my collection of 100mm Hitech Formatt resin filters. I'd previously tried doing this by getting an 82mm holder from Hitech, but the gap between the thread and the internal diameter of the holder is way too small to clear the XF300 lens hood thread.

As I have been typing this, it has occurred to me that I could have got round that problem by using something like an 82mm UV filter to bring the mount for the Hitech holder forward. I have only had the XF300 for a few months now, and it also occurs to me that a Circular Polarising filter might useful.

Anyway... I think I can see a way past all of the issues.

I'm using the donut made for my last camera, Sony EX3, it doesn't quite reach the lens barrel when done up tight, but no light gets in & the bars are strong enough to hold the box firmly in place.

Think your idea of a screw on filter, to provide something for the donut to grab onto, is a good one.

As a quick follow up, I sourced a 82mm UV filter, but it is 'slim-line' and doesn't help a whole lot as far as locating the donut. When looking for the filter on our NZ auction site 'TradeMe', I found the following two items that I thought might do the job:

A short lens hood with 82mm screw thread
Lens Hood Normal Focal Length Metal 82mm | Trade Me (http://www.trademe.co.nz/Browse/Listing.aspx?id=569936570)

And a 82-86mm step-up adaptor
Step up Stepping Adapter Ring 82-86mm. | Trade Me (http://www.trademe.co.nz/Browse/Listing.aspx?id=570882090)

(These auctions will timeout in about five days from the date of this post, but will be relisted.)

The lens hood would provide a substantial location for the donut, but as standard, would almost certainly interfere with the mattebox filter trays. I though the step up adaptor would provide a stepped shoulder and make an excellent mount for the donut. As things stand, I have made a new donut that is a perfect fit for camera and mattebox, and I can't see there being any problems with light leakage.