"Improvised" is great! And reflectors are smart. I don't think that seedlings and plants care very much about "fancy" and "pretty".
The reflecting glare makes it LOOK like a lot of light!
At first I was going to speculate that the brown/necrotic leaf edges were caused by dry indoor air, but the way that box is enclosed suggests plenty of humidity. Is there enough air circulation to keep it cool?
BTW, I use sheetrock/gypsum board/drywall under my trays so that when I use a heating mat, I don't lose heat straight down. I mostly start seeds during cold weather, and that room is very cold.
