I am looking for window security grilles (ideally, some sort of wire mesh
system that can be easily removed during the day) for a window I have that
is 5' x 5'.

Just an idea...

Channel iron top and bottom, and galv mesh in a welded frame.

Slide it into place, and you can put something simple like a padlock
through the channel iron to prevent it from being removed.

If you don`t have enough access to allow it to slide out, channel iron
to the top, and angle iron to the bottom as a surface it can sit on, and
padlock it in place in a similar way.