Dont use rectangle triggers if your mission is for mplayer. They migrate for no apparent reason and can be unpredictable
With careful sizing and positioning, you can create very long thin borders almost a straight line with ellipse ones!
Dont forget to put an outer warning zone to warn players when they are approaching it. There is nothing more annoying than walking into a killzone and not knowing about it
Another tip, just have the outer warning trigger send a message to the player that triggered it, not global, this way, if he is trying to flank, his position isnt given away.
To do this use the following line
On Activation : if (Player in thisList) then { Titletext[format["%1 \n You are near a West Safezone \nHead south or die!",name(thisList select 0)],"Plain down"]};