One last time.
So, I just put that into Illy. None of your cross members in your words will blast properly. Many areas are less than 1/16" while others are less than 1/32". In the sandblast world, those tolerances don't exist, especially when so much detail depends on those sizes. Whether you blast on HDU, wood or anything else, the edges will never match up for your digital print, either. It will look mighty wonky, especially again... if this is going to be viewed up closely. All the stripes and outlines will never work. All the detail is going to be lost and when all this comes together..... you're gonna have an ugly 15" sign.
You want dimension/relief, but you don't want to listen to reason, so do it your way and prove me wrong. I have nothing to lose. Even some hobbyist doing this in his little sandblast box with tiny grit is not gonna do this.
This is why in your first post, your sandblaster friend said, it's impossible. Engraving or laser cutting will work, but let the color off/out. At that size, the color won't look good, unless you keep it all one medium. Completely all hand painted or a digital print will work. Mixing too many mediums on such a small canvas is gonna look bad. Trust me.