I've recently repurposed 2x Keurig cup holders (drawers) for Glock barrels and gunsmithing paraphernalia in my safe.

I've also found that I can use fountain pen boxes for holding barrels in the range bag (even metal cigar tubes if a microfiber cloth is wrapped around the barrel).

I also gut and modify old day-planners of various sizes for concealed carry of the Glock

I use a Maxpedition Fatboy to hold my conversion kit and barrels for my 23-19 along with a small 'smithing kit, lights and batteries. Whenever I employ the conversion, I just reverse the load-out in the MaxP. This also sits in a rather mundane day-planner when on the road.

For my AR, this is a very low profile case to carry it in. I've custom configured scavenged foam on the inside from a Pelican.