I have explored the whole range from very close where the field stop is clearly visible and kidney beans are happening all the time, to a long way out where there is significant vignetting and I have lost the outer 10%. I did this in 0.5mm increments using O-rings. At no point in this range was the edge of the FOV easily accessible.
If edge blackouts are happening no matter how what you do with the eyecups (I thought from reading your original post that extending the eyecups solved that issue) the only other things I can suggest would be:
- eye placement ie. where you put the binoculars over your eyes. 8x42s ought to be pretty forgiving in this respect, but it may be that something about your individual perception demands very careful eye placement. You have already stated that you've tried varying the IPD without success (I would recommend doing so again, but making very very slight adjustments as sometimes only a very slight nudge is needed - NB. the old porros with a scale showing IPD were helpful here; I've also found that on a day to day basis I sometimes need to adjust IPD very slightly for maximum eye comfort); if you are convinced that all IPD adjustments have been tried and will not resolve the issue...
- the only thing left, short of changing the binocular in question, is to try different glasses. This can make a surprising difference - both in the curvature/position of the lenses interacting with your binoculars but also how steadily the frame rests on your features: the steadier the frame is itself, and how it rests, the better. Incidentally, having a top-quality pair of glasses probably isn't a bad idea if you want to wring every last ounce of optical quality from your binoculars. I have admittedly been somewhat remiss about following that particular piece of advice myself, but I really ought to.
In my experience quite a bit of the edge in the FL is both out of focus compared to the centre due to the curved field and also slightly astigmatic. How much of it do you find useful?
Most of it. I've had the opportunity to compare the 8x42 FL with my brother's 8.5 Swarovski on multiple occasions and without a doubt the image quality near the edge is not as good - you really notice this after using flat field binoculars (using the Nikon 10x42 SE as much as I do does not help!). But after a fairly brief adaptation period I get used to it and it's good enough for me. It may be that my eyes' accommodation ability is still sufficient to overcome the FL's edge aberrations. I use other non flat field binoculars fairly regularly and get along well with them too.
It may be that in having started out with pretty average binoculars, from which everything I now use is an improvement, and having had the good fortune to try quite a few different binoculars over the years, some of which took a while to learn how best to look through them (for want of a better expression), I've had a fair amount of practice in adapting to binoculars. I'm very grateful for that.
PS. I think conditions can be challenging at this time of the year - poorer weather resulting in poorer light, and the low sun angle can make things awkward even on sunny days if what you're looking towards is anywhere near the sun. But equally there's no heat haze, or that very harsh summer sunlight...