Thanks Kevin,
I've already have made some slight tweaks to some of the files (queue-tickets-tmpl, user-lookup-tmpl, ticket-open-inc) but I know this isn't sustainable and it becomes a bit of a faff every time we update.
It's really ticket-open-inc that I'd be looking to edit here, potentially just adding in another <table> section to store an additional panel and then I'd just use JS to create an additional 'Internal Note' which would be used to link devices.
Is there a way to edit this form in a more system-friendly way than just hacking about with the file and replacing the hacks every time we update the software? This is what I was trying to achieve with a plugin but the guide I linked didn't really cover this kind of scenario