Fc321,, your post sound like you are stressing far to much about making the right choice..
Thank you for your posts and pics as they will help others..
As a friendly word of advice.. think not in terms of right and wrong,, but in works and doesn't..
factory battery was not working for you,, even if just cause you felt you could not trust it.
The 35 size and 24 size would both work for you, as does the scooter battery for the guy using it , they all work so they are all right choices.
Smile, relax, and thanks for sharing
Please post pictures of your modified tray in case others wish to upsize their battery when replacing