My local 99-cent store carries small tubes of RTV in red, white, black, and clear.
I would run some tests to make sure it sticks to everything.
Then, clean 'n dry the pieces with alcohol to remove finger oils.
edit:
If the RTV won't stick to your new plastic disc, put RTV on the feedhorn metal lip, press on the disc, then apply more RTV overlapping the rim on the top of the disc.
Should make a good seal around the edge.