Hi White Rabbit.
The backfocus is critical to making the flattener work.
According to WO you need a Lens-to-chip distance of : 66-86mm
The SBIG camera without filters has a backfocus distance of 17.5mm (according to SBIG).
Filters will add 1-2mm so not work considering, but your filter wheel will add distance. Not sure what wheel you have but if you look on the manufacturer's website they normally give you either the back focus distance or dimensions of the wheel.
Don't forget to factor in the adapters, they can be 5-10mm as well (if there are any).
So you need to make up at least 48.5mm between the flattener and the camera.
Your wheel will be about 15-20mm so 30mm roughly of extension is probably needed.
Something like:
http://www.astroshop.com.au/products...sp?id=MAS-005G would work.
The WO unit gives you a large range to work with so maybe go with 35mm for safety, up to you.