Made a quick remix of this for my use case. Added mounts for a MOSFET with 42mm by 52mm mounting holes and a LM2596 buck converter. Also made room for a Y-axis damper (physically tested with thing:3143102, also tested in CAD with thing:3488452 which should fit). Changed internal fan to 40x40x20mm and added a spot for a 40mm intake fan. Moved Pi to the corner so all ports are accessible and changed its mounting to standoffs instead of posts.
Also included front feet to print out that fit under the extrusion if you didn't have any (like me).
Thanks to Mvieleers for the excellent design.
Products that I used (affiliate links)
SKR 1.3 board ($23): https://amzn.to/31wbrni
MOSFET ($10 for 2, only used 1): https://amzn.to/2WnWTUI
TMC2208 drivers ($22 for 5): http://s.click.aliexpress.com/e/cbGvRtw0
LM2596 buck converter ($0.69): http://s.click.aliexpress.com/e/9CDnpkc
Heatsinks for the Pi, SKR, and stepper drivers ($6): https://amzn.to/2XiOndl
MOSFET (~$3): http://s.click.aliexpress.com/e/caKv4TYM
TMC2208 drivers ($35 for 5): https://amzn.to/2W0oG1j
Edit 6/2/2019: added a version Box Left_4010.stl that fits the stock 10mm-thick 40mm case fan.
Edit 8/12/2019: added a version with thinner walls 2mm instead of 4mm as suggested by user mellda (Box_Left-thin.stl, Box_Right-thin.stl). Currently untested but should work fine and save some time printing.
Voltivo ExcelFil ABS