We have a poorly designed downspout that has a 12-inch horizontal section
that begins about 8-inches down from the roof; the 12-inch section appears
to be welded at both ends We've noticed rainwater backing up onto the roof,
and discovered that roof gravel has collected in the horizontal section over
the years. Tried washing it down with a hose, but that didn't work.

I'm considering drilling a hole in the horizontal section (sort of a
cleanout) so I can push the gravel down through the primary long vertical
section. I need to then cap the hole, but want to keep it accessible.

Can anyone suggest something to use for a removable cap? Thanks.