This is a modified version of https://github.com/sul-dlss/iiif-cropper. This is not concerned with the plugin itself, instead, this repo is just a way to quickly host a tool to interactively select the region of a IIIF resource.
To use it, find the IIIF Image URL of the resource you are interested in. For example:
https://dc.library.okstate.edu/digital/iiif/OKMaps/3904/info.json
Then take that url and replace <url> below with it.
https://okstate-maps.github.io/openseadragon-cropper/demo/?url=<url>
For example: