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I have my printer since december last year, so I'm also still new to this (definitely not an expert).

I would just print out a holder and if it is to snug than just print out another one with a slightly bigger dimension. For my printer, I need to add around 0.3 mm to have a nice fit (so if my smartphone is for example 70mm wide, then I enter 70.3 mm in customizer). Maybe for your printer it's different... You will have to test it :-)