There's a few ways to add elimination zones, but most arenas just use this bit of code from the tabletop arenas:
def Tick(self):
# check to see if anyone has been "eliminated" by falling off the tabletop
for each in self.players:
if plus.getLocation(each)[1] < Y:
plus.eliminatePlayer(each)
return Arenas.SuperArena.Tick(self)
where Y is the height a robot needs to be in order to be eliminated. If you were able to figure out the .gmf, finding the correct Y value should be easy enough.
The code is pretty flexable, plus.eliminatePlayer(each) can be changed to any command, such as plus.addPoints(each, X) or plus.force(BotID, X_Direction, Y_Direction, Z_Direction) or whatever. You can also change the < to a > if you want a robot that is higher than a certain height to have an effect, rather than lower.
This thread also has some information on how to add OOTA zones and other hazards.The arena itself is looking pretty good. The only thing I would really add is a hazard or two, just to give it some variety. I feel some combat zone-esque popup spikes, or hellraisers would fit well with this arena.