Helmet fitting and size

Seems an easy thing to get 'right' but I'm having issues getting a road specific helmet to fit. My head is an odd shape - it's naturally aero
like a peanut with a bulge at the rear. This means that back of the helmets can feel like they dig in a little. If I wear a medium sized helmet correctly, I'm at the point where my head at the back feels unprotected, but the forehead is properly fitting. I can tighten the rear adjust snugly by a single twist and the helmet looks like the right size.
If I then try a large, they look massive on me and im generally tightening up the inner clasps and rear to nearly full! I do feel like the rear of my head is adequately protected this way though?
I also read somewhere that when in between sizes, go for the smaller, tighter fitting.
Helmets I've tried so far are Specialized Evade, Giro Synthe and I've bought a Bontrager Ballista with MIPS in Medium.
I've spent over £700 is helmets so far and all have the tags in!
Any advice?
Thanks

If I then try a large, they look massive on me and im generally tightening up the inner clasps and rear to nearly full! I do feel like the rear of my head is adequately protected this way though?
I also read somewhere that when in between sizes, go for the smaller, tighter fitting.
Helmets I've tried so far are Specialized Evade, Giro Synthe and I've bought a Bontrager Ballista with MIPS in Medium.
I've spent over £700 is helmets so far and all have the tags in!
Any advice?
Thanks
Ribble Stealth/SRAM Force
2007 Specialized Allez (Double) FCN - 3
2007 Specialized Allez (Double) FCN - 3
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I have the kask protone and rate it v highly. I'm usually on the cusp of M/L and find that the M in the protone fits me with adjustment either way.
But would recommend a shop visit,a bit of fuel on a trip is going to cost a lot less in the long run.
Looks like you really having issues. Same as me having issues with most sun glasses.
Unfortunately I can't be much of a help but can't help wondering what you did with those helmets with tags still on?
Maybe you need some kind of custom made helmets. Not sure they exist.
I've just been notified the evade in medium is ready to collect. I'm trying these on at home to compare in my own time. Most shops don't seem to carry much stock so the best bet is to order from the usual suppliers and return for free.
It does seem that most road helmets leave the back of your head slightly exposed?
2007 Specialized Allez (Double) FCN - 3
Hmm I have POC helmet with MIPS (and a lot of other ones too including Kask) and POC does seem to give a good back coverage.
What helmet do you wear now ?
2007 Specialized Allez (Double) FCN - 3
...otherwise hit the LBS and buy from them!
Well I guess its probably most likely that you'd hit those areas - but it would be an odd crash to hit the bottom of the head thats not protected by the helmet ? It can't be too bulky there or it'd get in the way.