One method that a lot of Green Tree Python keepers have used is to line wooden cages with self-adhesive shelf liner, specifically the brand Contact Paper. Depending on how rough the plywood is you may need to put down some primer first.
Another choice would be clear vinyl film, although I don't know if that fits your definition of "non-toxic". Not sure if a layer of Sintra would be truely non-toxic either.
One method that I have recommended a lot here is to use something ultra durable for the floor, like an actual layer of plastic or epoxy and then a waterborne urethane for the visible walls of the cage.
Or you can spend a bit more money and coat the whole interior with either a bartop epoxy or a clear, non-blushing laminating epoxy.
It all depends on the look you want, your budget and whether you want a set-it-and-forget it cage or don't mind doing some touchup and repair now and then.
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Currently keeping:
6.10 Gonyosoma oxycephalum (Javan, mixed colors)
1.1 Philodryas baroni
1.1 Lampropeltis triangulum multistriata
1.0 Rhodesian Ridgeback