If you also make sure the bottom wire is properly earthed then they will get a serious zap when they connect the two. Respect for the wire is what keeps them in.
Reels of string tape or rope make the best temporary fences with a few intermediate posts at corners and a lot of poly posts to support the wire. You dont need a lot of tension for rope or tape, a lot less than wire, and reels make it easy.
Look at Rappa fencing stuff. Their steel posts go in and stay in but can easily be moved around.