With Rainsford's vast background as a hunter on virtually all continents of the earth, I think he would be an excellent travel and adventure writer. His various kills, from the Cape buffalo to the Amazon jaguar, would all make for good reading. Rainsford seems intelligent and well-spoken, so with the help of a good editor, I'm sure he would be successful with this new profession. He could certainly recount his story about Zaroff's "most dangerous game" on Ship-Trap Island, although he may have to change the final scene when he kills Zaroff.
Rainsford would make an excellent game warden in a big national park in a country where there are many wild animals and most of them belong o endangered species. One of the things that Rainsford would be very good at would be coping with the poachers who are destroying so many endangered animals, including rhinos and elephants. He would not only be good at that work but he would enjoy it because it would suit him perfectly. Lions and tigers are also endangered species. Rainsford is clever enough to trap animals to be kept in zoos where their species can be preserved and nurtured.
I think he would be a great scout or leader of a hunting expedition. He knows his animals, he knows how to survive, and he has already written articles as Zaroff has read some of them. If I were a hunter, I would want him leading my expedition. He also would be a great lecturer on wild life, hunting, and the dangers of the wild.