From BR Bullpen

A bandbox is a slang term designating a small ballpark in which it is easy to hit home runs. The designation was most famously applied to the Baker Bowl in Philadelphia, PA, although many other parks have been so called, mostly derisively.

In common parlance, a bandbox is a cardboard box designed to hold a hat. The term was quite common in the beginning of the 20th century although it has since become almost obsolete.