The guys above are right on; sealant on the vertical surfaces and a tiny dab in the corners of the horseshoe seal. I've replaced mine a few times in 18 years, and I always put the round seal in the cover first, rather than trying to install it last, without having the special tool. Another good trick I found to make the front cover slide easily over the horseshoe seal is to lubricate it with spit!
Spit is real slick, then dries up; so it works pretty well - cheap too.

DG