I wanted to use Solenoid's excellent Raspberry Pi and camera parts for the Printrbot Simple Makers Kit (1405), but that design is only compatible with the Raspberry Pi B - whereas I wanted to use the B+ I had on hand - this remixed adaptor replaces the one in the original thing.
As with the original, no additional screws should be required - simply undo the 4 screws on the side of the Printrbot and install the B+ mount. The screw holes for the Pi are sized for M2 screws (as the Pi uses M2.5, but I figured M2 are more common - the holes can always be made larger).
The B+ has annoying placement of the microUSB power connection, facing upwards towards the Printrbot's y axis. This mount should offset the Pi far enough away from the base that the power cable won't interfere - but you might need to cable tie it down to prevent it getting caught by the moving axis.
Update: Due to the annoying placement of the B+'s microUSB port, it tends to interfere with the Printrbot's y axis when in motion. While an adaptor or right-angle cable might get around this, I decided to move my Pi to the opposite side of the Printrbot to avoid the issue entirely. A new design to fit the different screw pattern on this side is now uploaded. This design 'flips' the Pi, so the microUSB port still faces upwards (instead of into the table), and the USB ports face rearwards (to wrap around to the Printrbot's USB port. Check the "bPlusHolderLeftSide" files for this updated design.