Summary

Re-use your old 9V DC battery casing and terminals.
Use 4 - 2032 coin cells together in series, then arrange the 2 sets of 4 in parallel for 12V output. Good for the LED strips, G4 Bulbs etc. with good quality batteries, there is close to 2,000 mAh
Make sure it doesn't get mixed up with a regular 9V.
CR2032 Coin Battery measures 20 X 3.2 mm.