I have been building my own housing for years, and there is only one glue for sticking plexi.
Bostik tensol 70. This is a 2 part glue. it welds the glue.. I have had housing to more that 50m using this. Very stong. You buy it from a good perspex holder..
Silicon is good for sticking flat ports on flat surfaces. Use silicon designed for the job, rather than bathroon silicon. You can get it from car shops, it is used for instant gasket.
It is designed t be used in a marine enviroment. They make a tensol 14 or 40, this is ok, but the 70 is better.