Belly Band is the only solution. I made mine out of 4" wide flannel (doubled). Measured Charlie's tummy, added enough width to accomodate the lap-over which has velco. Placed a Prevail Extra inside and it is perfect. He doesn't spray when I am home, but did mark his territory when he first came to live with me.
I work during the day, and the belly band is secured around him before I leave. No more worries. Here is the link and an easy fix.
http://www.dogchatforum.com/belly_bands_dog.htm