I have built a custom source module. I followed the steps for coding and did a make, make test, make install. Then I did the gr_modtool makexml my_mod, and a 'make install again. I can see the module in /usr/local/share/gnuradio/grc/blocks/my_mod.xml. When I run gnuradio-companion I can find the module when I search. I am running gnuradio-companion 18.104.22.168.