I Made The Dumbest Mistake On My Dutch Tool Chest. Advice???

Sooo, I have a small predicament…I am working on a Chris Schwarz Dutch Tool Chest Build and I am building it via a video he made and his pdf plans and I FORGOT TO ADD ONE OF THE TWO SHELVES!!! He presented the small chest but I built the bigger one. I don't know how I missed it but I only put the lower shelf in!!! I think it's because I didn't see it in the diagram…DOH!!! Also, I'm very sleep deprived with four kids…Lol. Ok, I'm making lame excuses now. Anyhow, I didn't cut the dados for the top shelf and I already nailed on the top front piece with cut nails. I don't want to remove them now. How would you suggest adding the upper shelf in behind the top piece? I thought about nailing in from the sides and front. I also thought about adding "runners" (I think they're called that) and just dropping the shelf in on top of them and then nailing in from the sides and front. Your help for this very amateur woodworker would be much appreciated!!!

P.S. Pay no attention to those locks. They are way too narrow and leaning. I promise the slots aren't off. Just had to throw that in there to make myself feel better! Lol.