Been using 3M mastic tape for years.  You can pick it up at any Home Depot or Lowes.  You can cut it to any size you need & almost never falls off. 

I never glue the flashers on because if you need to pull them off to service the equipment it looks like crap sometimes when you put them back on.  Plus if the cable box is defective & needs to be swapped out the cable guy won't tear up the flasher. 

Not a fan of installing the flashers inside a device either.  Tried it for awhile a few years back.  It requires more labor & can be quite a pain in the -ss depending on the device.  And same as above the cable guy will tear up the flasher resulting in a service call.  In my "opinion" this is a bad idea & usually causes more problems that it solves.

Web link is below:

solutions.3m.com/wps/portal/3M/en_US/3MElectrical/Home/ProductsServices/ProductCatalog/

Thanks,
Matt