Since my personal MakerBot should be up and running soon, I thought I take a stab at tweaking this awesome design a bit. I added a little stability with some feet out to the side (which eliminate the need for additional M8 bolts to hold the thing together) and added a comb made out of 1/16\" PETG to put tension on the spool so it doesn\'t spin freely and unravel itself.
The wheel components are cut from 1/4\" acrylic, the stand parts are cut from 1/4\" ABS, and the tension comb is cut from 1/16\" PETG. You can find some recommended cutting settings on some of my other things (thing:806 and thing:772). After cutting everything out, assemble the bearing clamps (3 M5x25 bolts and 3 M5 nuts each) and glue the captive nut and head recess plates to their respective clamp plates with acrylic glue. See the photos for details.
After the bearing brackets finished setting up, install them and carefully sandwich the bridging pieces between the two wheels (which are cut out of each wheel itself). Then feed a 14mm section of M8 threaded rod through the bearings and install a washer followed by 2 M8 nuts on both sides. You should tighten these nuts down reasonably tight, as the wheels can flex a bit, causing the bridging sections to pop out while feeding plastic on to the spool, which happened to me on the previous version that I built for our hacker space.
Next attach the stand. Assembly of the stand should be pretty straight forward, just add another M8 washer to each side of the M8 threaded rod and another M8 nut to each side after poking it through the large stand plates. I also added some lock washers at this step because I didn't want the threaded rod going anywhere as I went to town on the spindle as I spooled plastic on to the thing. Next install the M5x25 bolts/nuts into the T-slots in the bridging sections of the stand.
I would suggest you populate your spool with plastic at this point and not install the tension comb until after you're done. Don't put so much plastic on the spool that it bulges out past the edges of the wheels of the spool. After installing the comb with M5x15 bolts and washers your spool should still glide smoothly when you turn it but the plastic shouldn't unspool and cause the spindle to spin when you let go.