Many years ago I made a fuse/switch panel to a boat. I used 3 mm aluminium, because any thinner or steel was no go.
I used 22 mm floor grade (extra strong) MDF as a sacrificial board. I drilled the fixing holes (that actually were used to fix it in the boat) and used them and panel printed out of transparency to mark the holes.
Inside the cutouts and over the holes I put thin doublesided tape.
MDF was prepared/clamped and one side had a strip of wood epoxied and milled straight. The strip was needed to index the panel straight before dropping it down. Good luck trying to straighten panel with double sided tape......
I used plenty of screws to clamp down the panel.
I milled the cutouts plenty of trough, but left generous "tabs" on longest sides. Then I screwed down the cutout, with a screw that had a large head, some unscrewed portion that was excactly same diameter that the slot I milled. After the hairy bit I found out that you need two screws on every four side of the square cutout. Phew.
There were some blemishes, but luckily I was working on the back side of the panel, none of them showed up on final work. I left some cutouts with the tabs, because It was not too hard to cut the tabs out and file straight parts.
Needs plenty of clamps to support thin sections between the cutouts. I used plywood.
Round holes cut with camfer bits came out perfectly camfered and clean.
I cleaned up the face of the panel with solvent and abrasive sponge. It came out nice.