I've done 3 mantels in the last 20 yrs, a federal, a rustic, (log cabin) with live edge with no visual support. And a simple box style, rectangular columns, bed molding, dental molding and a faux 2" thick shelf. Once I let the lady down, I don't have a lathe and most of the moldings she was squishy over she didn't want to pay for when she found out how much the molder knives were going to cost let alone the cost of the sugar pine or the actual work to mill it. The lumb. yds didn't carry them.
For the box and federal, I did a flat mock up with ply at the home with the owners present then took it home and built it to the mock up in 2 pieces. It took some scribing and mat removal on site, but they came out nice, the custs were happy and the mantel looked like it grew there.
Work smart not hard!
Never bite the hand that looks dirty