Help I need bracket to mount MAilbox on Granite post

My wife recently picked up a really cool granite looking mailbox post and I'm going nuts tryng to find a suiatable bracket to attach the mailbox What I need is an ornamental shelf bracket thats about 12" long per side one side on the post one beneath the mailbox

I've checked every sort of hardware and hobby locally and can't find anything like this

I see mailboxes mounted on Granite post all the time this way so I know they exist would appreciate any help finding a source

Also this granite column is actually a 54" plastic sleve that fits over a

Thanks in Advance

plastic sleeve to mount a mail