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I think whether you need sky flats or can make do with t-shirts/light box etc depends a lot on your optical system.
With my fast Newtonian, a light source anywhere close to the tube has some light paths angling directly into the focus assembly near the top of the tube. These light paths are very different to what I get with a sky flat or when imaging.
I have tried t-shirts, light boxes, EL panels and more but the only way I can get a flat close to what I need is doing sky flats. It is indeed a PITA but I'm convinced it's what I have to do. I still can't get good flats with my HA images but that's a different story..
I shoot 30 mins before/after sunset with the drives turned off. Star trails show up so it takes several flats to get a good median, and ideally moving the mount either side of the pier and around a bit.
With long refractors et al. I think flat boxes etc are more likely to be ok since the light paths from the sky or something close in front of the scope are the same - all straight down the tube.
Whether sky flats are what you need for your CDK I'm not sure, but I'd be interested to hear what you find out.
Phil
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