(Update, if you want you can find a way to attach the laser to the print head of an etch a sketch and not worry about this part. Here's a demo on how to take apart an etch a sketch)
First you need to use a 3/8" drill bit to drill a perpendicular hold in your scanners carriage. Fit your 3/8" rod in it and hold it down firmly with a pipe bracket. Do the same for the other scanner
Cut your Aluminum tube into two pieces of 3".
Take the block of wood you cut to 2" by 2" and use another bracket to affix the 2.5" tube to the top side. This is your X directed slider.
Then use another bracket to affix the other 3" tube to the lower side perpendicular to the first. You can see the pictures or the movie to see what I mean. This is your Y directed slider. A breif plotter demo is at the end of this video:
Fit your Rods through these sliders and ensure they are all perpendicular to each other. Raise one scanner off the floor 1" so that the slider is not sitting at any angles and can slide easily in both directions. USE LUBE.
Once everything is arranged and slides just so. Open up your old school windows PC and it's time to get postscripty!