We do a rather large backpack and secure it to the back seat. We put it on pretty good with bungee cords and caribineer locks in addition to the backpack straps. We have a back pack where the zipper pulls can handle a small lock, too. That backpack isn't going anywhere with anybody with the way we do it.
My suggestion is to secure the backpack empty onto the
ECV. It probably takes us 20 minutes to do what we do to get it on. We keep it on; so, we don't keep having to secure it.
I love having a backpack and a basket.
You can on some scooters get a back basket that's larger than a front basket. It's considered an accessory if you want to go that route.
There are bigger purse bags that you can buy that have clip-on straps that are very long like a camera bag and this could be secured on the chair seat for a very easy on and off storage bag. For that matter a bigger camera bag would work, too. It just depends on how much you want to store.
Keep in mind extra weight on the
scooter does subtract some from the battery power. Probably not enough to make too much of a difference, but it all adds up to draining the battery.