Cliff Hartle wrote:

Screws in the end grain of a 4x4 post don't have much holding
power. It wouldn't take much of a push at the top of the box to
pull out the screws.

Plus, I'm thinking there could be some structural issues here.
Was that (relatively) small square area at the bottom built to
handle the strain (just pure weight and wind) of supporting the
whole box stuck up in the open air like that? I would think
it's an invitation to come out some morning and find the entire
top blown off and the base plate still anchored to the post,
but again I can't see the construction quality from here.

You could screw a 12" length of maybe 1/2" threaded rod into the
end of the post and use a nut and a very large thick washer
inside the box. The mesh bottom shown in the lower picture may
weaken the bottom and may make it hard to mount it this way.

If OP insists on mounting it that way, I agree that there has to
be a stronger way. Maybe "build out" the top edge of the post
with some mitered 2x6 wolmanized pieces that would give a wider
base and a bit of a stronger nailing surface? That would make
the top edge of the 4x4 roughly a 7x7, and he wouldn't be
screwing into end grain if he stays out around the edges.

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