You can just snap the 9V batteries together in series. Snap the positive of one into the negative of the other, then snap the positive of another into the negative of the 2nd one and repeat with 10 batteries. You will have open terminals at the end of the string. You can take the terminal board from a discarded 9V and cut it in half to make connections to the string. Here's a photo I found on the web to illustrate what I mean.
