Anyone recall a book where kids had a club and dressed up like famous super heroes?

[Gargleblaster]

I recall three of the kids dressing up as Superman, Wonder Woman, and the Flash, but there might have been others. I remember reading it when I was younger (probably 20 years ago) and I can't remember what it was called. I do recall that they had a clubhouse (or was a tree house?) that they called the Fortress of Solitude. Anyone else remember this book?

 

[Unregistered]

Did you find an answer to this?

I have been looking too. It's driving me nuts.