Try making a heat sink to protect that plastic. Then "tin" the bottom of the sight, and "tin" the location on the barrel where you want to solder it. When putting it on the barrel, wrap the barrel with a wet rag, to act as a heat sink to protect your finish. Then clamp or hold the new sight in place, and instead of heating the sight, heat the barrel, until the two thin layers of solder melt together. Remove the heat, and he solder cools almost immediately. Use graphite, or some oil/grease, or other release agent on the barrel to keep solder drips from adhering to any part of the barrel you don't want to solder.
It doesn't take much to pull the heat away from that sight. Any metal will do the job if you can attach it between the bottom of the sight where you are going to tin a thin layer of solder, and that plastic insert. You might contact the manufacturer to find out how to take that apart to temporarily remove the plastic insert.
Almost 40 years ago, I used a similar tube sight made by Weaver Optics, on my shotgun to learn consistent mounting, hold and follow-through, when shooting clay targets. When your head was aligned properly to look at the plastic Orange insert, you saw a glowing orange light. If you head moved away in any direction, the light went out.
I credit that sight for the very fast improvement in my Trap shooting scores, shooting far fewer targets than everyone else I shot with back then.
I sold it to someone who was training his teenage son, and could not find the sight for sale. Its nice to see that similar sights( even with different color inserts) are now available for all kinds of rifles( See a Cabela's catalog). Tom Knapp, the TV famous Exhibition shooter, is using a similar sight on his shotguns, and advertises them for sale on his website.
TomKnapp.net.
