You maybe going a bit overboard in the jig/fixture department. I usually try to wait until I come to a point where I need a new jig/fixture before I make it. Reason being, plans, techniques, and direction may change, and you tie yourself to what may not be the optimal solution.
I used for a light box, a drop ceiling light fixture with the diffuser for a light box. Inexpensive at a home improvement store or electrical supply house.
For a straight edge, the cheapest one you can buy is off-cuts of ALCOA Mic-6 cast aluminum plate. You can find drop-offs on eBay cheap and in most any size, and they are Blanchard ground flat to at least .004"-.005" over the entire surface.
Still for jointing soundboards, I'd probably shy away from using sandpaper and go with a hand plane instead. Two or three deft strokes can get the center joint super tight. And since the wood is sheared not abraded you should end up with a stronger joint, which looks cleaner under close inspection. I do use a sanding block for some back joining, but it doesn't apply here as I use it for some figured back woods that I fear may tear out due to the figure, like bird's eye or curly maple. I don't even use an expensive plane; a cheap big-box-store plane works fine.
Looks like you're all set up to go! Good luck!
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