Thanks for the suggestions!
I have the 168C and it is similar in construction (so far as I can tell) to the 268C and also includes in the box the centre angle adjustment adapter with its female M54 thread interface. On the QHY web page for the 268C specifications it implies that the CAA is optional but doesn't give any examples of how to connect to the camera without it. On my 168C, the circular dovetail flange (which the CAA is tightened on to) is also threaded - for M54 it appears although I've never tried. However there doesn't seem to be much depth to the thread and the heads of the 4 screws that secure the dovetail flange to the face plate of the camera protrude a couple of mm and may interfere with any attempt to attach an adapter. If you can do without the CAA then you save 6mm backfocus and if your filters are 3mm thick you have to add 1mm (one third the filter width) to backfocus so you would then be spot on, assuming you didn't need any additional adapters (e.g. thread converters). If you have anything with a male M54 thread then I guess the first step is to try screwing it into the dovetail flange of your 268C and making sure it can fit securely and measure the thread depth for your backfocus calculation.
For my 168C, I use a Cyclops Optics BladeC adapter to connect the camera (with CAA) to my Canon lenses. It can hold a 2" threaded filter but it wouldn't be compatible with your filter slider. They support the 268C too, may be an option to consider if you can't source an alternative solution.
I unfortunately have not been able to find any way at all to use the QHY268C without the CAA, and I have read online that it is a compulsory component, meaning that you must sacrifice the 6mm. I have discovered a way to thread a two inch filter using the given spacers and some M48 extensions. I have attached a picture of this setup below for anyone else having the same struggle.
It is good to know about the Cyclops Optics BLADE-C; thank you for pointing that out! This may offer slightly better modularity if it is easier to change the filter without disassembling the entire spacer assembly.
My current QHY268C/EOS lens back-focus solution:
Not sure I understand your comment re filters - you only need 1 filter: an LP or dual narrowband filter will also cut out the IR.
I was probably unclear about the filters; sorry about that. I will need at the very least the UV IR-cut filter, or any of the other filters I mentioned, but definitely only one.
Overall, I have realized that it doesn't currently seem there is any solution out there to incorporate any filter drawer with this camera and an EOS lens. The additional 6mm of the CAA is devastating towards this cause. This may be the biggest difference between the QHY268C and the ZWO ASI2600MC Pro, since the ZWO ASI2600MC Pro allows for threading directly to the front flange, and there is even a dedicated filter drawer EOS adapter that would be absolutely perfect... If the ability to change filters while using a camera lens without disassembling the all your spacers (which will take quite a lot of time in the dark) is valuable to you and some of the added features of the QHY268C are not, I definitely recommend you get the ZWO ASI2600MC Pro.
If anyone else has thoughts or comments please let me know. I have a lot to learn!