I've taken custard filling off of my list of choices since I've had two cake disasters and they've both been custard filled...one was only a 6 inch round. They just sort of settle and slide. The layers were all level and I damned it before I filled it but it still slid. I know there are bakers who put 1/4+ inches of soft filling in with no issues, what is the secret?
Well, I'd speculate that you put in too much filling. It can't be above the dam.
Here's a link to recommended cake filling amounts:
(A 6" would require 1/3 cup.)
I make custard filling pretty often. I make sure not to go above the dam. I also make my custard a little bit stiffer than normal and make sure the cake is well refridgerated before pick up or delivery.