Gravel in Downspout

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.

Here you go. Google Expandable rubber plug. Lots show up. WW

I'd try roto-rooting it first to break up the gravel. Probably stuck together with dirt and leaves. Once clear, the gravel should stop collecting and a wire cap on the top to keep leaves out?

If you can get up high enough to drill a hole and then fill it, you chould be able to use a coathanger wire or similar to put it down the frain opening and wiggle it while applying water pressure from the backup in the gutter. It wil be messy, but necessary.

A piece of properly sized hardware cloth which will prevent the roof gravel from passing through which has been cut and properly shaped to fit snugly into the drain pipe...