[eluser]richthegeek[/eluser]
you are posting to entirely the wrong address.
POST sends to a HTTP web address... NOT a file in your filestructure. That means it accesses the controller/method in exactly the same way as a human visitor would... so you want to post it to "/makeCanvas/saveBoxes", allowing for subfolders if required...
BTW: for all Javascript development, you're gonna want to use the Firebug plugin for Firefox - the "Console" tab will log all AJAX transactions (what was sent, what was received) and turn bright red when an AJAX send 404's... if you had used that you'd have seen in approximately 3 seconds that you were posting to somewhere that didn't exist, although not sure it'd have helped get around the brainfart you were having...