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Nice work! I've made several cajons of different designs playing with snare positions and guitar wires and they all seem to sound pretty good. I use 19mm ply for everything but the tapa which I uses some ply I bought for laser cutting which is about 3-4mm thick and perfect for the tapa. There is one change I would recommend to your design though and that is the screws at the top for securing the tapa. I don't put in as many screws at the top and I place a small piece of card under one of them to slightly raise the wood from the frame this allows you to get a really nice slap when hit near the corners and really adds to the dynamics of the cajon. Terrible audio, but this video shows what I mean..https://www.youtube.com/watch?v=Zm_bj12veKY