A good epoxy, deacon, or marine tex. I’ve heard of guys using s carbon fibre arrow shaft for weight, but I tend to use a piece of 1/4” key stock. Just have to mill out the plastic cross-pieces. You may also want to use some epoxy to glue the rod, etc. Into place, before filling with epoxy. Depending on what you use, it will sometimes float in the epoxy - ask me how I learned this. Do not fill with epoxy to a point where it will touch the barrel, unless you want to spend a bunch of time sanding it out to free-float the barrel.