I don't know if this the right place but:
I have also made one custom widget plugin for Designer but I have a troubles to implement it (use it). It appears in designer I can also move and place to MainWindow form. Compilation is OK but when I want to run I get this error:
"error while loading shared libraries: libworldtimeclockplugin.so: cannot open shared object file: No such file or directory"

The path to the library is correct because of "make" run without errors.

Please can you post here a short example of how you use the widgets in designer? It would really help me a lot