I was wondering if there is a foolproof way of getting a chaos gate to spawn in the mourning fields or do you still just need to get lucky with 7 rerolls.

You do just need to get lucky, although you can have up to 10 rerolls (either starting with 10 and using none or using up to 2 and getting them back from Narcissus). Rolling 3 times makes it about a 1 in 2 chance I believe.