Middle seat belt for kids booster seat

[LordDevlin]

Hi there,
Did plenty of searching on the forum but can't seem to find what I am looking for. My son is now large enough for a booster seat and the wife says that she now wants him to be in the middle between us, rather than when he was against the window in his big kids seat and she was stuffed in the middle between us.

A simple backless booster seat will allow us to still have some space. But to put him in the middle I will need a shoulder belt in that middle position. Just wondering if anyone else has done this at all.

I have a small window 59.

Any help or advice would be appreciated.

Regards,