I happened upon an interesting Web page at ThinkZone. Someone wrote a program to create portmanteau words based on overlapping letters and then listed "the better ones." It's a long list.
Technically, I guess they're portmanteau words, but they lack that certain naturality. They say that necessity is the mother of invention, and so it is with portmanteau words. The words in this list are motherless children looking for a home. Sure, some of them may be used sometime (badvantage and playpenitentiary aren't bad), but when they are used, they'll be reborn naturally -- not pulled from a list created by a program.