Here is how I would do it.
For a light, cut a round hole to match a ceiling octagon box in the tin. Now get a couple of pieces of 1x6 and edge join them. Use pocket screws or some kind of brace across the joint and use water proof glue on the edges. When it is dry, cut a matching hole in the center of this. Using stainless steel screws drill and screw though the tin from the outside into this board with the 2 holes lining up. Use at least 4 screws. Now the board is mounted securely to the tin.
Your octagon box will mount in the hole, but you need a ceiling bracket like this to hold it in place.
Then install a ceiling light cross brace
(frequently these come with the light fixture).
Because most tin is not flat, you are going to have to figure out some kind of a gasket to go between the fixture and the tin. To get fancy take some more 1x6, cut 2 pieces so the are square (must be bigger than the base of the light fixture) and glue them together. Cut another hole through the center. Gouge out the back side to match the groves in the tin. Paint it
(2 coats of spray primer and 2 coats of spray finish) to match your fixture or the tin and screw it through the tin into the wood backer. A bit of caulk and your done.
A lot of work, but it will look nice and last a long time
