So I should scuff up the back piece, right? As it stands now, it is transparent and I think roughing the surface with some sandpaper will help diffuse the light.
Honestly, I think roughing up the surface of the back piece is a mistake. It will block more than diffuse the light, and introduce colours that aren't in your art. What you want to see when you look at the marquee is light, filtered through the colours on your marquee art. I rather like your art a lot, like a Monet it is an outdoor scene full of natural light, colour and activity. You don't want to be putting a grey wash through that.
I once did a few Mortal Kombat themed cabs and got some very nice marquee art printed, designed to be backlit. I then experimented with clear perspex, paper, and that translucent white diffuser I mentioned. Then, because I didn't quite trust my own eyes and objectivity, I took photos of the results so I could compare them side-by-side. The translucent white perspex backing won the competition easily, then colours were clear and bright with the lamp itself not visible at all. Done right, the white paper behind looked acceptable in 2nd place, lamp not visible but light/colour was dulled a little. Plain clear perspex didn't diffuse the light at all, uneven bright spots obscured the details in the art, lamp was essentially "visible" and, to my eyes, looked barely acceptable. I didn't try scratching but refer my comments above for why.
I wish I could produce the photos from that test here and now, but it was many years ago now and was only done for myself to understand. Even if I could find the photos, I couldn't be sure of the right captions/descriptions after all these years, so you'll just have to believe me
Javery I'm curious what you're planning to light the marquee? A LED strip? A simple 1 inch deep cardboard box behind it with the inside painted white and a led strip around the back wall should light that up easily enough. White surface reflection works wonders
I made a cardboard box but lined it with aluminum foil. Zeb also said I should have covered it in white paper instead so I might do that. I bought a USB powered LED strip for $8 from Amazon (the kind with the peel off backing to stick it to a surface) but it's not long enough. At only 18" I can only span the marquee width one time. I should have bought a longer strip but the 36" strips are only in soft white (not cool white like I wanted). I am returning them and might take a drive to home depot today to see if I can get something there.
IIRC, what I actually suggested was painting the inside of the box with some plain white paint. This will reflect just the visible light. "Ceiling white" is ideal if you have it, but otherwise (and thinking $$$) whatever leftover cans you might have from painting the house or something will look fine, so long as it is at least mostly white (most wall paints are actually off-white, which is why I recommended ceiling white above). Remove the foil, because it will look too harsh (shiny, reflects images, not just light) and will reflect pretty much everything except gamma rays back at your balls, and don't use paper (feels like a fire hazard).
As for the LED colours - cool white includes more of the light spectrum and will highlight the sky part (blues, violets) of your marquee more. A warmer light will highlight the beach parts (reds, orange, yellow) more. Whatever works for you!
I do love your work on these cabs. The marquee is a standout, a unique part that you want to emphasise for that feeling of an Avery family beach holiday that you want to capture.