This flattener has three expansion rings with two spacer rings separating them which slide into the telescope nosepiece and expand when tightened. Unfortunately the 2" bore in GT81-IV nosepiece isnt long enough (approx 24mm) to contain all three expansion rings which in my case resulted in the innermost ring popping out and ending up in the main tube.
A workaround is to disassemble the spacers and rings and place two expansion rings next to each other: expansion ring - spacer - expansion ring - expansion ring - spacer in that order.
Trial and error has got me close to round stars across the whole frame but slightly off in one corner.
With perfect alignment and all three expansion rings held tight over their full length as originally designed and supplied, I think this flattener would be totally fit for purpose.

Wasn't sure what to expect - adding a new piece of glass to the setup often seems to cause problems or lead to disappointment. Well this flattener has been astonishingly good! I'm using an F7.5 ES 127 apo and with the flattener I am getting a truly flat field - good tight, round stars to the edges of an APS-c Nikon sensor. It seems very well made and the squeezable rubber rings arrangement holds my DSLR very steady and, as far as I can tell, right on the optical axis with no evidence of tilt.

Works great for my star wave f5.9 152 refractor and my Nikon d850 when bought with the T ring adapter
Sharp images with only minimal vignetting
Highly recommended !!!
Can’t wait for more clear nights to use it more
Bloody weather !
Still first results on m81 and m82 are good from just a few pics as cloud came over on my first and only test but sharp stars all around and good detail.
My shots of the moon have come out amazing so much crater detail
Very happy with my purchase
Buy one!!!!

A long hard search for a flattener that would work with my F6.9 William Optics Megrez 90 led me to try the Hotech SCA field flattener after a fairly dismal performance from the WO FF/FR III.
The Hotech gives me great star shapes to all edges of the frame when using a dslr whereas the WO left a lot of curvature through the frame accentuated badly in the corners.
I can now image cropping only for stacking artefacts.
The self centering mechanism is a nice feature too not only for keeping the imaging train central but for rigidity, the flattener will remain in place in the focuser without the thumbscrew securing it (I found on removal and would of course always use the thumbscrew too! However the extra reassurance of the centering device providing a lock is great)
The interior of the tube is very dark and the general construction quite robust.
The only flattener that comes close to this quality of performance (from other Megrez 90 users) is the TS flat2 which is considerably more expensive and requires further attachments.