It's a little clunky as I'm not that great a designer, I also used parts that I already had around (like the power supply). Still, I think it may meet your requirement of a "Mac Classic using the tech of today". I've since added a proper video card (RTX 3060) and a nice cutout for the rear ports.

https://twitch.nervestaple.com/2022/12/19/pc-in-macse-build/

While not as portable as a laptop, it's nice to be able to pick it up and move it around.