Heater Core leaking already!

[429mustang]

I just replaced my heater core when I bought my car early this year, I could smell coolant every once and a while so I replaced it before it got bad figuring I'd be good for another 5-10 years. Yesterday I noticed some green drops on my carpet while rolling my window down - F**KER! It sucks cause I'm getting ready to move next week and it's not like I don't have a bunch of other things that need to get done. I won't even have to pay for the $18 part since it's under warranty, it's just a b*tch it happened already. I even made sure not to buy the brand from autozone that I heard was no good. I used blue RTV before I set the core in place, is there something better to use underneath the heater core to hold it in place?