Many of these mailboxes are deisgned to be bolted or tied down to a concrete plinth or foundation. This is just a matter of boxing out a suitable area where you want the mailbox installed and filling it with post mix concrete. The tie down bolts can be set in the concrete (even while in the mailbox) while wet and then tightened after the concrete is set. In many cases these can also be installed on a suitable concrete paver (available in most hardwares) by using masonary anchors. See the description for the Moderna and Vectra which have a link to their own installation instructions.