I've used motorcycle helmet liners before they work well and sit snug around your head and it looks pretty sharp inside the helmet https://www.ebay.com/itm/332079569957
Just cut the plastic piece off and glue it in
But as Mullreel said the Kevlar padding works great too and it's nice to be able move them around as you may need.
If you wanna keep it crazy super accurate, they just used a basic suspension rig in the original prop. Probably ripped from a hardhat or something.
Otherwise, some cheap fabric foam or EVA foam likealexmiller1 mentionedhttp://www.thedentedhelmet.com/members/alexmiller1/ is a pretty common solution. Most comfortable though would definitely be a MICH or bike pad set like Mullreel and jonbenm linked, but it might be a little hotter in there.