Why, oh why, are you mucking around with a lightbox? A 16” sphere can be had for around $40 shipped… http://www.smoothfoam.com/product/10073.html
Also, there is not much need to worry about external light getting into the sphere. My sphere sits on my kitchen floor. It is not covered or lined. The kitchen is lit by 10,000+ lumens of LED lighting. Around 0.5 lumens of stray light makes its way into the sphere. Not likely to be a problem except for very low level lights.
This is my data acquisition system: Any interest in a LED/Battery analyzer device?
The control software can compensate for background levels by subtracting the minimum light readings (i.e. the background light) from the data. For the real finicky tests, I turn off the lights. I can also mount the sphere light sensor onto the outside wall of the sphere and use the sphere wall as an attenuator. This increases the maximum measurement capacity of the sphere from 5000 lumens to around 600,000 lumens.