I have my ASI1600MM-Pro setup two different ways on my Celestron SCT. One "straight through" at f/10 which will have artifacts at the edges/corners of the frame, and one with an Starizona SCTIII focal reducer / flattener (pricey!). I tried the Celestron f/6.3 focal reducer but could never get good stars across the field, no matter how I adjusted the backfocus.
First for the SCTIII, I have an imaging train like this:
Then for the "straight through" option I have an imaging train like this:
After considerable tinkering with various connection schemes involving visual backs, clamps, etc, I am now a firm believer in fully-threaded imaging trains. Since I took up this practice, I've had much fewer incidents of sag/tilt. Between Amazon, Agena Astro, and what comes with your ZWO camera, you should be able to get the miscellaneous pieces to put it all together.
I've also found a spreadsheet and a digital calipers to be invaluable to double check my math.