Mine is a 10x10 electrical box with a single layer of duct seal (right in pix, behind the target)1 layer is enough for low power 10m guns.My secondary is a spinner silhouette trap (left in pix)Target is taken to the trap via a DIY target carrier with a piece of cardboard as the target backer/holder.This cardboard has to be frequently replaced, especially when shooting with a rifle, where the shots are more concentrated than with a pistol. Once the cardboard is shot away, leaving a big hole, it becomes useless in supporting the target to make a clean hole.As opposed to Marty, I have an extreeeemly restricted vertical height, under the house. The crawl space is literally a crawl space at that end, so the target carrier is a practical requirement.
I use the back of my neighbor's garage....