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.

The front looks great IMO. Kudos. I have a few compacts myself and one project I need to try is tossing a Pi 5 in there and having it display on the original CRT. Would make for a cute little SSH terminal.

The most impractical Mac that I'd love to see reviewed is the iLamp, aka iMac G4.

You can buy reproduction MiniITX-compatible cases on AliExpress, but then you're likely stuck with whatever China-quality LCD they put in it.