If you are operating in daylight, at a range of 150 meters, you will absolutely need to use a telescope (collimator) of some sort.

Let's look at power. Your cube size is about 1/8 of a meter, and assuming a brightness of sunlight (about 500 W/m^2 for visible light) it emits about 8 watts of optical (visible) energy, and we'll assume is does so isotropically. At 150 meters, a hemisphere with that radius will have an area of 141,000 m^2, so your power density at the detector will be about 55 uW/m^2. Assuming a detector size of 1 cm x 1 cm, this will produce a total detector power of 5.5 pW. This will be a challenge.

Another way to look at this is to consider relative sensor areas. Let's say your detector will look out over a 10 degree cone. At 150 meters, this cone will look at $$A = 4 \times \pi \times (150 sin(5\deg)^2) =  2,148 m^2$$ All of this area must be assumed to be emitting or reflecting light at the same power level as the cube. Since the cube only has an area of 1/8, turning the cube on or off will only produce a detector difference of about 1 part in 140,000, and this is much less than you'd expect the background to change due to random movements, tree branches moving in wind, clouds moving, etc.

So the first order of business is to look at the cube with a telescope - in other words, to use what you call a collimator. The more powerful the telescope, the better you reject background changes and the more power you focus on the detector.

Once you've stepped up to a (fairly powerful) telescope, how do you reject the changes in background? If you can turn your source on and off, and if it is something like an LED, you can turn the source on and off at a kHz rate and perform what is called synchronous detection, also call a lock-in amplifier. I'm not going to go into this, but you can find lots of information on the web.If you're going to build something from scratch, try the AD630 modulator/demodulator chip.