The first questions that need an answer are:
How will you get the speaker wires into the porch ceiling?
How is the ceiling of the porch ceiling framed?
If you had the ceiling material installed (looks like it may be aluminum, judging by the seam at the corner), was the old material covered, or replaced? If it was replaced, maybe you have a photo of the work in progress, to use as a way to jog your memory and make running the wires easier.
One way to get the wires to the speaker locations would be to run the wire up from below in a corner that's somewhat hidden from view when looking from the front of the house and from the photo, it looks like this could happen around the corner trim at the right of the door. Once the wire is high enough, a hole can be drilled upward and into the space above the porch, so the cable can be fed in and brought to the closest speaker location- you'll need a tool often called 'fish sticks' because you can't push wire if it's limp, like this stuff (these sticks are available at Harbor Freight and they're not expensive). Because this kind of framing is usually constructed of 2x material in a triangular form, feeding cables should be easy once the cable is there and the speaker holes have been cut. See the image for what I mean.
You would then use the fish sticks to feed the cable from hole to hole. Because the distance between the speakers is so large, I would run the system in mono because drastic use of stereo separation and panning would be very annoying and this results in a 'ping pong' effect WRT the sounds bouncing between the channels- listen to old Beatles records for some good examples. Another way to avoid this is to use dual voice coil speakers that connect to both channels- you will need to use an impedance maintainer in order to avoid ending up with a speaker impedance that's too low for the amplifier and unfortunately, they waste a bit of power. This could be OK if you don't plan to listen at high SPL, but in reality, 45W isn't a lot of power when outdoor speakers are involved, even at moderate levels- you are trying to fill a large area even if that's not the plan because the perimeter doesn't have a wall to contain the sound.
If you decide to use some kind of small box speaker, I would think about putting them below the deck if the boards have a decent gap between them. If this isn't the case, they could be mounted at the outer edge of the ceiling and facing down at an angle, toward the house. Getting the wires would be similar, but they don't need to be fed above the ceiling material, just coming out at the point where the 4x4 and ceiling meet (drill from the porch side of the 4x4). Then, the cable can be stapled at the corner and fed to each speaker location and covered with some kind of moulding.
