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I think all you need is the 5mm or so spacer to be honest.
The SBIG works, and it is "about" 17mm. The Flat4 is designed with the DSLR spacing in mind, so 44+/- and 10+/- for the T ring, about 55mm again +/-.
From the comments about, the QSI without the nose-piece is about 50mm back-focus, so the 5mm will suffice. If however the QSI has 5mm back-focus WITH the nose-piece then things change, but only by 50mm.
Assuming the 5mm spacer is what you need then the fine-tuning could be done easily with the Flat 4, it has adjustment between "66mm and 86mm". I usually use mine at about 76mm, the mid-point, but at least it can be fine-tuned to get the best star images at the corners.
Gary
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