SpVgg Unterhaching, currently in the 3. Liga, have a capacity of just over 15k, and while they often suffer as their suburban München status means they potentially lose fans to Bayern, they are well supported.I went and watched Unterhaching a few years ago in the 3rd tier and they probably had a similar gate to a League One side.
Over half the the capacity of the Sportpark Unterhaching is „standing“ terraces, and these are generally very well populated (a colleague of mine is a fan of Die Haching, she is from Unterhaching originally).
A proper decent old club, one who has graced the Bundesliga in seasons past.