I would protect the box with the buttons, the rest should be safe in the rain (maybe even that is waterproof but I would still do it to be sure). make sure you recheck and hand tighten all plastic nuts that seal the wires, and having wires on a downward slope for a small distance from those nuts and not upwards should make sure water doesn't try to find a way in constantly.
That being said cold rain on a heated lamp is not so good for durability so if possible I would put a layer of greenhouse plastic film (on a frame or something) a foot or so above the lamp. That should make it super safe, and probably as durable as running it inside.