I am currently Building the awesome "Ultimaker 2 Aluminum Extrusion 3D printer" http://www.thingiverse.com/thing:811271 and found the uppermost X bar of the bottom-construction could tilt -> that sounded cryptic -> The bar that holds the lower part of the Z-stage: Z-motor and the lower portion of the two 12mm rods. The brackets are 18x18x18mm and are meant for M3 counter-sunk screws. I have not assembled all of it yet, and the tilting might be a passing issue, when everything is done.. I decided to forego any issues and wanted to Mount corner brackets on all the bottom-upper-joints (hope that make sense, but see Photo). I thought it would be easy to find some corner brackets, but it wasn't (they were too big or had other wrong Measurements for my project), so I just designed some myself. I used the Measurements from motedis.de and made 2 different models. One with guides and one without. They Work exactly the same, but the one with guides are a bit harder to print, unless you have good cooling (I don't). Do yourself a favor and print them solid. There is no real material gain by skimping and it is annoying to see your corner-brackets bend or the screws going through them. I printed some in both 0.1mm layer height and some in 0.2. Works fine. The model with guides must/should be placed as it is (pyramid like) to get all details, but it might cause some issues internally.. Play around with it.. maybe even use a tad of support.