The professional variants of malicious USB cables can fit a whole GSM in there: <a href="https://boingboing.net/2017/11/15/cottonmouth-for-the-rest-of-us.html" rel="nofollow">https://boingboing.net/2017/11/15/cottonmouth-for-the-rest-o...</a>
Im impressed that he did that with a mill. I’ve used a laser printer, uv box and acid before. I think the setup cost less than $100. Easy to mess up fine tracks however.