How can/should I mount this porch light?
A pretty common approach is to build a small platform for the lamp to sit on, something like this



This has the advantage of giving you more depth to mount a receptacle box. You can use 3/4 inch material (nominally 1x ) or larger, such as 5/4 stock. You can then cut a hole, using a hole saw through the new block and the siding underneath giving you enough depth for a much deeper box, like this one



It also has the advantage of making it easier to get the base of the lamp vertical.

After the box is set, I would caulk the entry point of the wire and around the box itself. After the lamp is mounted, unless the fixture has its own gasket, caulk around the top, sides and all but a small section of the bottom (a weep hole just incase any moisture gets in.)