Probably the best way to handle this is to make a really stiff icing dam. Sugarshack adds lots of PS to her icing to make it stiff enough to roll with her hands, then uses that as a dam.
Alternatively, you could dig out a layer of cake from the middle of the cake, leaving a rim of cake to be the dam. Sue Tickel does this....she uses a thin knife to cut a circle into the cake surface about 1/3inch in from the edge, then uses an offset spatula to carve out the circle she just cut. Then fill in the cavity with filling. I've tried it, and it works very well.
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