The local chandlery carries a complete supply of barrel nuts, but they sure are not cheap.I believe their supplier is a company called Fasco:
Fasco Fastener Co Inc
2041 Clement Ave
Alameda, CA 94501-1317
(510)522-4049
Yes, the screws do go up from inside. You can see one in the liner just past where the grab rail ends (near the winch plate)