I read a lot of web pages. Possibly even on this forum. There are two products that I saw recommended constantly. One was a 3M adhesive called 5200 ($19 a tube and the complaint was getting it out of the grease-gun style of metal tube). The other highy rated product was Lexel, a clear adhesive in a clear tube, like typical caulk gun plastic tube ($9 at Lowes). The main complaint was can’t read the fine print on the clear tube. I bought a tube to seal a 3" neoprene foam bulkhead I made to replace a trashed one in a used Tsunami. (It looked like a hungry badger got trapped In the rear compartment and tried to claw its way out. In point of fact, the hatch had been replaced). MI forced the new bulkhead in place but waiting for a cool day to crawl around and do the job.
Research seems to indicate that silicone caulk is the common bonding agent used; however, multiple sources and technical explanations indicate that silicone WILL NOT stick to the roto molded plastic (if that’s your material). If silicone was used, the sources recommend cleaning the area with something like acetone, then abraid the area with 60 grit to aid with a mechanical bond. Actually, I havent done it yet, because I dread the idea of crawling around in the tight space more than I hate cold, boiled mutton. Sorry no more to offer. Hope that at least give you a place to start your research and a possible source. One person said he was looking to see if they’ve made any advancements. One user who recommended Lexel said, “Why? It’s been working fine for ten years, why do you need something better?”