Thermostat 2005 hyundai elentra

I took the old thermostat out which was really bad and stuck open. Now i cant get the housing back on because when i put the new thermostat in its place and try to put the housing back on is falls back out just a little causing a big leak once i get the thing together. I tried the rubberband and paperclip trick and tried putting a piece of tape on it but it still falls out. please help i gotta get this fixed today.

Put a small amount of Permatex #2 on the backside of the thermastat flange. Wait until the Permatex is nice and tacky and it will hold the thermastat in while you assemble it.