I've just attached a flip-flat to a new Tak FSQ-85EDX. The flip-flat has a spacer to allow it to work with 4" refractors instead of the larger ones supported by the larger panel size. When mounted, it's very close to the edge of the light panel, as below.
My question - is the panel illumination consistent enough across the entire field that this won't matter? I'll definitely give it a try - but it could be hard to disambiguate between illumination changes and vignetting along the edges.