Buy a small plastic humidifier. Not sure how much that will set you back, but it can't be too much. Running 20% RH is probably not where you want to be for any extended period of time. Where is that cupboard anyway? The middle of the desert?
If my metric conversion is any good, that is a really, really small space. Considering that, 27 degrees C, is not too bad. If you have a way to cool the space even further, you may well see the RH increase, although, I doubt it will come up much from the arid air that now fills the space, without a humidifier.
Your light is underpowered if anything. I run a couple of the 400W HPSs in my flowering area & it is necessary to augment their output with movable 400W MHs. The light spectrum is very good for budding, yet left to itself, you may find the flower structure & size to be lacking, due to an insufficient amount of illumination being delivered to the grow points.