So I did some more messing around with the rails last night, it's impossible to find K&S angle brass in this town!
I did, however, find some Plastruct (stupid expensive for what it is!) and Evergreen shapes and some rectangular brass tubing.
There's an online store, hobbylinc,
Plastruct I Beam ABS 5/8 (3) Model Scratch Building Plastic Sheet Rod Tube Strip #90031
that's based here in Atlanta, but there's no storefront. I ordered some angle brass and some shapes in grey ABS, so we'll see what that's like. I wonder if it was grey ABS that they actually used (leaving no need to paint the white styrene), since it's available to model builders in the same shapes as white styrene.
But I managed to cut some new brass rails and tried to figure out what they could've thrown together to do the reveal box with the styrene I got.
I didn't take pics when I was finished, but I will this afternoon. I ended up adjusting my models and printing new plastic rails, and filed the brass so each L was about equal in length and width. It turned out looking better than my cut-up C channel, so I'm happy with that.
Also, the newest version of printed rails holds the card pretty snug, so there's very little wiggle, and it's a good pressure-fit for "revealing" the box guts. Not too loose, not too tight.
Promise I'll take more pics later!
This last pic shows what I think whoever built the rails might have done. Use one T section to hug the corner of the box, and another smaller one to create a track for the card. Part of the smaller T section could've simply been cut away, making an L (or they could've simply USED an L), but I don't show that here. But my model is basically that, if you break it apart into separate shapes.