Mono DSLRs are all about the filters and how you manage them. Given the backspacing requirements of the Canon APS-C sensor, I chose the Teleskop-Express EOS 36mm Filter Drawer device:
http://www.teleskop-express.de/shop/...w-Profile.html
It allows me to use my current T-Adaptor and I got the 36mm filter drawer sent with it (ask on ordering as they can swap the standard 50mm, and 36mm filters are slightly cheaper and provide plenty of field for the APS-C sensor).
The issue that arises from putting a filter drawer on the front of a cold finger cooled DSLR (one that is using a dry gas purge system) is how do you prevent the loss of your gas and the internal condensation that might result. On the colour DSLR I just stick a Baader UV/IR Cut filter on the front of the T-Adaptor and it's fine, but on the Mono the filter drawer will leak and so needs a gas barrier behind it.
My solution was to use an Astronomik MC Clear EOS Clip-In filter to create the gas barrier. Normally a Clip-In filter is a press fit into the mirror box opening but to seal it I used black polyurethane on the outside ( the idea being that I can remove it if I had too in the future). Importantly, that MC Clear filter must be spotless before you install it because it is not a casual removal to clean it. Obviously don't get any sealant on the outside surface of the MC Clear as it will be hard to clean off.
Now the Filter Drawer can be used without loss of the internal dry gas, and stops any dust from getting into the shutter area from the aperture of the camera. You can always clean the outside of the MC Clear when required.
Photos attached.