I have a cake on order due this Saturday. I only just yesterday brainstormed compatible fillings for the cake flavors my client chose. 

The top tier is, what I call, Autumn Mixed Nut Cake. It has walnuts, almonds, pecans and hazelnuts. It is fairly sweet so I thought mascarpone would be the perfect flavor profile for the filling. 

I almost always go with Italian Meringue Buttercream variation for my fillings cos it's such a reliable, sturdy component for tiered cakes. 

I worry that simply adding mascarpone to an IMB will make it much looser than I'd like and not taste enough of mascarpone on top of all that butter. 

Therefore I'm in need of either a reliable mascarpone IMB OR a fantastic and sturdy mascarpone cake filling recipe. HELP!!