Goddamn man.... you made THAT, and you're asking US how to mount the Arduino? I'm pretty sure you're in the top percentile of people on these forums when it comes to fabricating enclosures.
I always put mounting holes in my PCBs (I don't use official boards or clones very much, because I have a severe case of chronic not-invented-here syndrome), but I just screw nylon standoffs onto them and sit them on the desk like that. You're saying that it you want to have the board *rigidly attached to an enclosure? That's just crazy talk. Next you're gonna tell me that devices that need to be above the floor and behind furniture shouldn't just be dangling from dupont line or something....
But seriously, mad props on the enclosure, that looks beautiful. The only thing I want to suggest is.... try searching AliExpress for combinations of words like "metal" "panel" "Indicator" "LED" or "switch" and so onthere are some astonishingly cheap, really slick looking panel-mount LEDs, buttons, and switches; depending on your aesthetic tastes, you'll probably love them or hate them compared to the ones you're using - so it;s worth checking.
(Or ebay, you see the same stuff there - but aliexpress is cheaper and more importantly, has a much wider variety. They ship via ox-cart (with water buffalo to cross the pacific) but if you place like one order a day and don't need anything you buy there urgently, you still get one package a day, so it feels almost like you got it overnight. Only it's even more exciting, because by the time it arrives, you won't remember exactly what it is, so there's a christmas morning sort of effect every time the mail comes. Just don't buy bare IC's supposedly made by western companies on there, or anything expensive enough that you'd particularly care if it vanished or turned out to be be halfassed crap.
Coincidentally, aliexpress is THE PLACE to get nylon standoffs and screws for mounting boards. I use M3, they fit in the same holes that I;d use for #4 screws in imperial measurements, and they are cheap and abundant. Prices on there are just in another galaxy compared, for example, to McMasterCarr. Ther quality is usually at least mediocre (I'd buy screws and standoffs, and try to avoid using the nuts from there, they tend to be disappointing) - but do you care how strong the standoff that you're using to mount a circuit board that weighs under an ounce is?