Have one that style back in the store room collecting dust that was never installed due to last minute customer design changes. It's all aluminum. Sort of the same construction as channel letters, but with thicker material.
The face and back are just panels cut the same shape, the outer is bent from a single piece with a bent lip on the bottom, notched in the corners to shape it that the back panel sits on, attached with rivets, and the seam where the ends meet is riveted. The face is screwed to small L-brackets riveted to the sides on the inside so you can remove it. They're pretty solid once they're assembled. The lights on this are just standard edison sockets that retro looking LED's were screwed into, whatever light socket you want to use, just cut the right size holes to accept them spaced evenly. Plenty of room inside for wiring, and lighting controller if you're using one. If you're using several individual letters/ pieces and want to control all with a single controller, you can mount them to a raceway for wiring, or design them to be attached together (use bracing on the back side to keep them solid) and put holes/ grommets to run wiring through from one to the next. You can attach the back panel with L-brackets like the face if you don't have the know-how or equipment to form the outside panels with a lip on it. If going interior you can use just about anything for the face & back panels, but aluminum is still best for the sides, because it'll hold it's shape better.
Now that you reminded me it's there, I think I'll clean it up, hang it up somewhere in the building & light it up.
Cheers