Is it necessary to enclose a raised play area?

I want to make my windowseat a playpen for the guinea pigs for their daily out-of-cage time. It is about 2' off the ground and I was wondering if it's necessary to fence it in? I don't want my pigs to get hurt but it's an odd shape and it's difficult to enclose.
If anyone has any ideas on what I could use to enclose it that would be helpful too.
Thanks in advance.