Depending on the available space for wiring connections behind the light fixture itself you may not need a box. Instead, you can install a siding mounting block specific to your siding profile with a wood block (2x4, 2x6 etc) behind it to back it up and support the fixture.
Wood block screwed to the exterior plywood, mounting block covering it, hole through all for romex. Fixture mount bracket will then be screwed through the mounting block surface and into the wood backer and connections will be made in the space behind the light fixture.
Code only specifies connections are not to be made inside of wall cavity. This type of installation always passes electrical inspection here in VA.