Safari Keyword Search
Firefox has an extremely useful shortcut that allows the user to query a search field displayed within a specified URL. Search is invoked by entering in the FF address block a user-configurable keystroke combination, followed by the search terms. For example, within FF I can enter "gi volcano" and FF will enter "volcano" in the search box at "images.google.com" and display the result; it's very quick and no intermediate screens (like the Google Images search page) are displayed. Keywords are stored by navigating to the desired search box, placing the cursor in the search box, and right-clicking to display the setup dialogue. The shortcuts are saved in a folder within FF bookmarks.
I find two Safari Extensions that approach this idea, but each isn't quite complete: CustomQuest can only use the preselected search engines; while Omnikey allows the user to specify the search site and keystrokes, some sites just don't work — one can search maps.google.com, for instance, but not images.google.com.
I'd like to use Safari regularly, but this search function is hard to give up; I have a dozen or so shortcuts set up and they're very useful. Are there other Extensions that do this? Or perhaps there's something outside the App Store. Any ideas?
MacBook Pro, macOS Sierra (10.12.6), null