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20mm + 3mm - 8.68mm = 14.32mm --> attached an y-axis spacer of 14.32mm

(you said you have a gap of 3.75mm, how accurate did you measure that? This spacer is 2.88mm smaller. If you actually need 3.75mm smaller I could make it but I'm afraid the belt might not fit the smaller gap. I don't know if the belt fits for the attached version either. I have just altered the size, it is not tested in any way. If the belt hits the damper then this solution will not work for you)