3d printing is a good option, or injection molding. Gemini does the walmart letters that are an injection molded front and back, but they gather a ton of dirt at the base. Massivit 3d printers have translucent gels, but it's mighty expensive. I've routed out acrylic fronts and backs that got bolted together, but the face will hotspot pretty bad. Usually I'll see front+back (channel letter with clear back), or face+sides, but not often you see everything lit.