I was messing around with a friend on a map I had made, which had a few added box_xl 's in doorways, when someone left and the game became laggy. I noticed some spawn points where the boxes were, and became intrigued, and grabbed one.
You can move them around, gravity applies normaly to them, and you can set their spawn times / spawn at start. I have tried to do that in hex and sandbox both, but it never worked. As you may know, invisible boxes/walls, along with invisible spawns, save with the map. This is bound to be an easier way to position them in relation to other objects/scenery on the map, so I thought I would share

I found that if, off-host, you go into a brief black screen, the spawns become visible. You can achieve this easily by unplugging (or lag switching/standbying) the ethernet cable between your modem and switch/router, or between your router/switch (switch/switch ect...), for about 5 seconds.
Or you can just have it be really laggy and hope it works.