Good peanut lacrose ball for thoracic spine release?

Hi, I've been focusing on sorting out my posture as my back has got into a bad state from sitting hunched at a computer.
I'm currently looking for a peanut-lacrose ball - to work between individual vertebrae and unlock them a bit.
There are many 'peanut-lacrose balls' on sale on amazon, but most seem to be 6cm in diameter; which i can imagine might be a bit small for your spine to be able to fit comfortably in between each ball.
A lot of the reviews indicate that the balls are a bit too small, which concerns me.

Could someone suggest a peanut-lacrose ball that isn't too small? I've tried to DIY one with two tennis balls - but superglue didn't form a tight bond and sellotape ended up removing the gap in between the tennis balls rendering them unfit for purpose.
Thankyou,
B