User blog comment:MachineWolf7/Sneak Preview for, Portal 1: Final Confrontation/@comment-24061949-20140409220627/@comment-5955017-20140410161352

I've built a set of "wall" pieces which can plug the holes that make up the portals. There is a small handle on the back so people can remove it, and a set of "catches" to prevent them from falling out the front when sitting. The portals themselves have smooth edges so the plugs can fit in.

There is a picture on the project of what this looks like.