Ill try to remember to take a picture of the inside tonight. I used a spat ear defender on my steel helmet. Basically you just need to drill/dremel out the gap where the 'vent' plate meets the defender part (a 3/4" slit)
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Ear defender hole for chin strap.jpg [ 188.85 KiB | Viewed 1475 times ]
Then you tuck the chin strap though that. For mine, I turned the metal end on the strap sideways, tucked it through, and when it straightens out it naturally is too big to come back. So its pretty secure and doesnt fall out.
I believe many will simply tuck it through and then secure it to the inside of the cup with a screw.
The key is letting the strap dangle out from under the vent plate, but over the ear defender
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Helmet Lft Prof.jpg [ 126.7 KiB | Viewed 1475 times ]
Is that what you meant?