I'm going to just be using two X-Wing mats put together, but the crease will be less than ideal.
A friend of mine uses two 3x3 yoga mats. They're pretty heavy and sit well next to eachother for epic games. They don't let the minis slide and they don't slide themselves. It's not the most attractive-looking option, but it's still pretty good.
But what options are you all entertaining?