...I remember, after marking off, and drilling a 1/4" hole in the center, to start, I 'saber sawed' with a Thin Fine tooth blade, cuts that left metal on Left and Right sides, Even. But Front to Rear was Biased, More sheet, Don't remember. The Box was built with Mild Steel, and had flanges that were bent over the Bent down Original sheet. Extra Supporting Plates were Stainless Steel, see pics. All Rivets are Stainless Steel. Custom Hammered Fit. Don't forget to drill a Small 'Drain' Hole in the Bottom.
...I Should Follow My own Advise, as the Positive Terminal is very close to a Grounding. Even though there is a Rubber Insulation there. I Must Fix That!