Can I make a simple suggestion?....
How about instead of going thru the trouble and expense of fabbing up a linear rail or otherwise, why not make a simple and sturdy pivoting mount for it that secures to the column. If you make the mount real adjustable it would make for a simple alignment of the unit once it is in place.... Also if you engineer the pivoting mechanism carefully it will not be a large heavy weight hanging out the side of the machine and it could possibly swing in a specific arc that actually gets it out of the way of chips easier... Oh and by the way, I am JEALOUS as hell that you are to the point of making one for your machine. Being the modbus and electronics master you seem to be it will probably be a SWEET setup. I cannot wait to see an RF45 with a nice changer equipped. March on my freind...haha peace
Pete