So it looks like the light box DOES make a difference. Nearly everyone identified Photo #5 as being the one taken in the lightbox without the flash. (Anonylanf elected instead to identify the model in the pictures as coming from Star Blazers.) Here’s that photo again:
And here it is with just a little tweakage in Paint Shop Pro (a skosh of Clarity, and a pinch of Saturation):
So I think pretty soon I’ll try to build a slightly sturdier but still relatively cheap box, possibly out of PVC pipe. Thanks to everyone for helping me with this experiment, and thanks again to Nicolás and Kevin for their photo help.