I really wish I thought of this before ordering the first batch of PCBs. Well this is going to be part of the v.2 boards: Mounting holes for an ADXL.
Probably not everyone wants to mount a PCB on another PCB, but at least this would avoid the need to 3D print an ADXL adapter, since you can just use some M3 screws to attach the ADXL at the top of the PCB (the holes flanking the logo). It can be mounted on either side of the PCB and cleanly wire to the JST-XH connector.
This design is by far not final, as I'm also considering just adding 8 pins to solder the ADXL directly to the PCB.. we'll see.