Well. One idea is to get blank IO shield for the ATX case and then make a hole for keyboard din connector to it.

ATX I/O shields with an AT full sized DIN keyboard connector hole used to be a thing, but they seem to be vaporware now

Those are easy to 3d-print. Or just make one out of a piece of card board.

Way I have done this is I buy little nuts that standoffs will screw into. So instead of screwing my standoffs into the motherboard tray. I drill a hole for them to go in then secure them in place by nut on the back side of the tray. It works fine. You can even get nylon lock nuts so they don't back out or thread locker juice.

Reason for this is its pretty hard to line up perfectly and this gives you a ton of wiggle room to get it perfect.
I line up the board in the case by installing expansion cards and I make sure they are not twisting at an angle, up and down strait and side to side. Then i mark my initial holes.

I was thinking Dremel or other rotary tool, blank I/O shield, drill, and tap for making threaded standoff holes. As for mounting plates for a turbo switch, the aforementioned 3D printing may do, but a push-button toggle switch and LED shouldn't be hard to mount on a bay cover made of more sturdy materials.