Only tested on Chrome. Definitely won't work on IE.
Create a bookmarklet with the following contents:
javascript: + a url-encoded version of $('head').append('<script type="module" src="https://tstibbs.github.io/ui.js?latMin=51.6618&latMax=51.6942&lonMin=-1.8363&lonMax=-1.7299"></script>')
Set the lat/lon min/max to represent the coordinates of a bounding box - you will not be notified about aircraft outside of this box.
Note two additional optional params:
maxHeight: don't notify about aircraft above this altitude (measured in feet)direction: only notify about aircraft travelling in a direction within 20 degrees either side of this parameter.
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Run your bookmarket
Note that the screen will have to show the area you're interested in - if the lat/lon parameters are off screen then you will likely get no notifications.