Hi,
The reflective light box is a very good idea. Next spring I'll place the electric blanket on a table by the window...and use reflective material to focus the light inside the box. I also have tiny light sensors...so one thought is to turn on a set of grow lights in the event there is overcast conditions during day time hours.
Update on my broccoli. Grace and I were out of town for 3 days. I am glad to report that the plants we transplanted outdoors are doing well. We did place them deep as someone suggested. Anyhow, the leaves and stems are noticeably thicker now. We'll keep monitoring for further development.