Firstly each helmet seems to have different weather it hex bot in to the ear or bolt from the ear in to the helmet
When I made my last ESB interior , I got a strap and cup from a police riot helmet (well Kurtyboy actually got them for us) picked up on ebay for not to expensive , and removed the strap for m the riot helmet , top attach them to the helmet,First of I had a nut n bolt system holding the ears on so for the right hand side one of the screws for the ear I made longer so I could attach the strap which Id burnt a hole through I glued washers on either side of the whole on the strap and added a few more washers before screwing the nut on , the other side is a bit different (if your going scren accurate that is) as it has a hex bolt that goes from the inside of the helmet to the left ear, I think this is on ESB only, as PP3 has wex bolts in to each strap ,though you can just repeat the same method as the other side and have a good result ,