I customized the Customizable Plate Bracket by mightynozzle for the REV Robotics build system that uses 1515 extrusion. I wanted my FTC robotics team to be able to create custom brackets quickly and easily since we have a 3d printer in our meeting space.
I set all the parameters to match the REV Robotics bracket dimensions, so you only really need to pick a bracket and it will work. But the parameters can still be changed. For example, change the lengths and you can make extra long brackets. Or make a 2+ channel bracket to join two 1515 extrusions in parallel - I uploaded this to the Thing Files so you can see an example. You have lots of options!
Note that these don't have the alignment ribs of the REV Robotics plastic brackets. Originally I printed these with a "screw hole diameter" of 4.0 and that would allow the hex bolts to just drop in. I reset the default to 3.9 so the screw would be tight in the hole to make it more rigid in compensation for the lack of ribs. Of course, adjust to your preference.
You can also customize in OpenSCAD instead of using Customizer - it is faster and has a better preview. Here is how: download and install the latest development snapshot from here: http://www.openscad.org/downloads.html#snapshots. Then in OpenSCAD, go Edit > Preferences > Features > Enable Customizer. Now when you open the OpenSCAD file posted here a customizer panel will automatically load in OpenSCAD.
Make it strong with 1.5mm perimeters (which should eliminate infill for 3mm thick brackets).
If your printer+filament has a tendency to warp, then you will want to add a brim to hold the bracket to the print bed.