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You could use svg for example :see https://codepen.io/evantill/pen/JjaqYdjJust change the diagram url and it will fit.
A great article on positioning elements in svg :
https://www.sarasoueidan.com/blog/mimic-relative-positioning-in-svg/Do not forget to tell plantuml to use a transparent background :
@startuml skinparam backgroundcolor transparent Bob -> Alice : hello @enduml