It needs to be quite firm... you don't want it falling off again!
Scrape off all of the old glue from the plastic frames, and remove from the glass panel carefully. I left mine to soak in warm soapy water, then just rubbed my thump over the glass to remove the glue. Same with the plastic, except I used a metal scraper.
Then refit with your gap filling adhesive of choice - I used black RTV silicone sealer, others have used epoxy, carpet tape etc.
Ben.