I use a lot of 9 volt batteries for my toy projects and I scavenge the terminal assembly from an old battery to make the connector for the power supply. At the same time I solder a switch into the assembly so it is a one piece unit. Then I top it off with a nice layer of hot glue to give it some strength.
Note. some 9 volt batteries have a different construction method and this assembly cannot be salvaged.
Old 9 volt battery
Scavenged micro switch
Suitable gauge wire for your project