Thermostat Leak

Well, I thought I would replace the thermostat on my 2003 Camry before it failed. That seems to be a mistake. The way it is set up from the manufacturer is the o-ring goes into the housing then the thermostat. Problem is, the thermostat will slip down (since it sits vertical) and not stay centered. so when I bolted the housing back on, it leaked. So, I reversed the order. Thermostat first so it sits in the small diameter housing then o-ring but now have a slight leak. I tried using a silicone sealant to hold it in the center but that did not work ...Can someone please help?? Is there a special tool I need?

Thanks!

devo