When I built I found it difficult to make a plywood box spine that was straight. I've got cabinet making and carpentry experience so I was a bit annoyed. I finally clamped my plywood box (baltic birch 1/2"cut true on a very good tablesaw) to a 4" x 4" square steel tube I had and then screwed it together. Unclamped it stayed pretty straight. Much improved but what's the obvious next step?????

Your aluminum strong-back looks great,especially the way you handle the ends.
A thin walled rectangular steel tube would be a bit heavier, but a lot cheaper--less than $100USD. And they come in lengths up to 20'
If it did sag, you could support it in the middle while you reinforced it with plywood along the sides, or bolt on another or smaller length of tubing along the top to stiffen it. Once straight it would never warp.
I'll never use a wooden spine again. They work, but the worry factor is eliminated with strong straight stiff metal.