I would suggest a cheap led floodlight up to 30w. I am sure you can find a way to fit it in your box. Maybe cut a hole in the roof so you get the extra benefit of cooling it down better. I suggest using a 3000k or even better a 4000k spectrum all the way. With a maximum cost of 10 euros you solve the problem for good. I guess the price in dollars will be similar. You can find them in any DIY store.
Me personally, I use in my 90X90x160cm grow tent, one 100w 4000k led floodlight for the first 2 weeks and then 2 100w 3000k led floodlights for the rest of the grow. If I notice they need more light I add the 4000k floodlight again and that's it. Total cost of the 3 floodlights is 60 euros. And the result is unbelievably good. Buds are more colourful, more dense, more potent, much heavier and smell and taste much better. I have used any kind of lights availiable in the market, in many different set ups and grow rooms, boxes etc, but nothing has proved so rewarding and so dead cheap.