[Witchdoctor]

What might be effective would to take a 2 1/2" cap fill it with solder.

Put it into the 3" cap upside with the 3" pipe between the two
then solder the 2 1/2" cap to the inside walls of the 3" pipe to seal it off

That would give you significnat mass on the buisiness end ... I can illistrate if you don't understand waht I am getting at ....

As said earlier though 3" may be a bit large once insulation is introduced