The boxed design is good... however I only see one linear rail. Maybe I'm looking at your drawing wrong... but this may pose alignment issues under load unless the bearings (you are using multiple bearings on that single shaft?) are widely spaced on the carriage. You need the multiple points of contact to prevent racking. It also appears that the torque resistance normally taken up with a second linear rail and bearing set is going to be handled by your ballscrew. This is not good practice. The stepper or servo will not like the torque load in addition to the Y travel forces. Making that screw perform double duty is asking for problems.