beleuchtungschalttafel-3000L. I used this last night and it seems to work reasonably well. Still need to come up with a way to mount it because lighting from ground level typically doesn't work in a photo.
When lit, the light coming from it is white, 4000K and very diffuse. The light source size isn't as large as a full-size softbox; it's about the size of a standard notebook sheet of paper. It runs off battery power, roughly 40W, a small laptop battery will power it for about an hour at full power.
During the daytime, it has a very minor (barely noticeable or detectable) effect. It's most useful from golden hour onward, sunset, twilight, nighttime, etc. For night I'm getting a 3000K phosphor and trying to figure out a way to switch the two without much effort (the backgrounds in most of the night images I took look too yellowish).