How do I keep the filling from going all over? Will the whipped cream be stable enough to hold the filling in? Or what else can I do?

No, whipped cream won't hold anything in that wants to escape, sorry. What you need to do it thicken up your filling. Depending on what type, you could stir in some instant pudding powder or cook some cornstarch into it.

What she said.

I had this problem last night - I just made up a small batch of suuuper thick, sturdy buttercream that tasted similar to the filling, and piped a dam of that around the outside before I filled the layers to make sure the filling didn't seep out the sides. The buttercream needs to be really, really thick though, almost too thick to pipe

Thanks, I figured there was so easy way to go about it! Nothing too complicated though!

Thanks again