I quite like the idea of a finite universe with rubber-type boundaries, where once you get to the edge you bounce back. Like a billiards table.
So long as it's a four dimensional (minimum) billiards table, then we're in business. In fact, can we change it to a snooker table? I know they're the same thing, it's just noone fucking plays billiards, plus it's shit, while snooker is actually quite good.

4D snooker table universe. I like this one.