I have the Alu plate on my GSA used the plastic one for my 58L Trekker on my XR and it was loose and made a lot of noise. With the Alu plate it is quite and secure and quite a bit lower as the missus now wants a back pad due to the difference in height compared to the XR. But that may be the height of the back rack I did not have the metal for the other bike.
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