Based on Mathias' ADXL345 mount for the X axis, I thought I would make one that fit both X and Y, as you require either two accelerometer chips for bed slinger printers (such as the Ender 3) or one chip that you move between the X and Y gantry for calibration.

This accelerometer mount is to be attached on to the hotend mounting plate for X axis calibration, and the Y carriage plate for the Y axis calibration. I have included pics for both placements.

Note that due the mounting placement, the data you get from the chip must be interpreted correctly:

ADXL345 Data : Actual Axis (X Mount) : Actual Axis (Y Mount)
X : X : X
Y : Y : Z
Z : Z : Y
E.g. The Z data from the chip while mounted on the Y carriage plate corresponds to the Y axis according to the printer itself.

The use of the accelerometer for tuning the input shaper functionality of the Klipper firmware is described here: https://www.klipper3d.org/Measuring_Resonances.html

UPDATE: I have updated the piece to be smaller, more robust, and better tolerances.