I read this somewhere in the 1990s as a standard-sized paperback, probably about 200ish pages. The protagonist is a man on Earth who I remember was acting as a Batman-style vigilante, I think maybe funded by being a trust fund baby. I do remember that he was nursing significant bruises at the beginning of the book, I think with the implication that his romantic notions of crime-fighting were out of touch with the real world.
He gets sent to a fantasy world, I think summoned by a set of priests who are devotees of the Woodchuck, except the name has been put through the Punctuation Shaker with a bunch of vowels replaced by apostrophes. I think he's given the cloak, which can become rigid, which comes into play in the course of the novel, I think once as protection where he kind of turtles under it and once where he turns it into a boat. Unfortunately, I don't really remember what happens in the plot after that.
The title had the Woodchuck in it, and I want to say that "seven" came up in it. And... I just managed to find it, so I'll post it as a self-answer.