Hi, welcome 
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Well as far as I remember you could run any widget in “window mode” using the plasmawindowed executable like this in your terminal:
plasmawindowed org.kde.analogclock > /dev/null 2>&1 & , and set it always on top from there, hide its border etc. like a usual window (accessed with Alt+F3 for example), I guess you 'd have to further create some autostart entries as well.
The names of the widgets are pretty straightforward after org.kde. , you can find them under $HOME/.local/share/plasma/plasmoids/ and under /usr/share/plasma/plasmoids/.
I’m not updated if it is doable by tinkering with the config files there, someone else could know