I discovered the holes in the flange were drilled just the right size to be tapped at 5/16-18, but I didn't have any 5/8-18 bolts on hand. I *did* have some 8mm x 1.25 bolts, but didn't have an 8mm x 1.25 tap. Thankfully, my neighbor did so I tapped the flange and am using a transfer punch to lay out the bolt holes in the lid.