What do I mount this large mailbox with?

I have found the best method is to cut a board that is the size of the bottom of the mailbox, mount it to the post with screws and then screw the mailbox to this base with screws.

I replace my old mailbox by removing it from a home made mounting platform and mounting the new mailbox back just like the old on was mounted.

Hmm...mounting bracket? This mailbox is intended to mount to a plywood (or similar) base that is attached to a post. When I mounted mine, I screwed the plywood (cut to fit inside the base of the mailbox) to the arm of my post, then screwed the mailbox to the plywood using the holes drilled in the side of the mailbox.