I would opt to use a 1/2 pancake box 4" in diameter. The NEC code does not permit the canopy or empty space of a fixture to substitute for cubic wire space for wires larger than #16 gauge. And a 4" pancake boxes cubic space legally can only hold one 14/2.
Since you already have the wire conduit on the inside of the house I would also consider using something like this box to mount the fixture to.
After reading a lot of the great comments I wanted to update my answer with some new advice.
Because a 4" pancake box is limited on the cubic depth you would need a round extension. 1/2" will add an additional 3.3 cubic inches.
A 3/4 extension would add an additional 5.0 cubic inches
One 12/2 would require 6.75 cubic inches so a 1/2" extension would suffice.
Installing the pancake box and the 1/2" plaster ring would require 1" of depth so that leaves you only 1/2" of wood to mount the box to. Using three or four weather rated 1/4" wood screws would secure the box without penetrating the interior side.
As a side thought maybe you could only recessed the pancake box 1/2" and leave the plaster ring concealed behind the light box. From the picture it appears to have enough room to conceal the extension ring.
Keep in mind you will probably need to rotate the box so the mounting bracket that comes with the light will not interfere with the box or the extension rings screw holes.