Horn waterproofing

Hello all,
Recently , i have acquired a second hand channel master type 183.
The antenna was delivered with a horn feed (REF 3040661) but this piece have a broken waterproofing window (sorry for my approximative english)

What kind of material to use to repair the protection ?
What thickness ?
What kind of glue / silicone to use ?
Any advice is very welcome

PS : Don't worry for the bizzarre assembly, it's only to not loose somes screws and toric joints.
Regards,
Jef.