FALLOUT 4: I BUILT AN ARENA - Let's test it out!  

I actually really like the use of the "Unstoppables" billboard sign, it makes you think that whoever is running the settlement is taking settlers, dressing them up like Grognak and the Silver Shroud and is charging caps to see them get torn apart by ghouls. In terms of getting npcs to use specific furniture there is a console command called PFT or PlaceFurnitureTester, although I've never really messed around with it so I'm not sure if it works. There is also the console command SetOwnership , which might work as well (it's how Mama Murphy's chair works). I've never done much settlement building in game (although I do enjoy building locations in the CK) so you might have already tried this but instead of a circle if you built the arena as an octagon it might keep the npcs from slipping out.