Yes I should be able to put a helmet in it, which is good.
As to the lock, I have been thinking about it in the past few days.
I looked around for a suitable lock cylinder, but it seems like the available space is really short. Maybe a lock like this would fit :
https://www.amazon.com/Prime-Line-Products-Defender-9941-Different/dp/B002FYERHG/
If you get them keyed alike, you can have the same key for all your locks.
However it is a very low security cylinder and it will most likely rust over time (apply ACF50 regularly to extend its life).
I thought about 3D printing an adapter to be able to use the same type of
cylinders than on my BMW bike, which should be similar with other brands.
My main concern is that due to the way the locking mechanism is designed, it cannot be secure. It can be opened with a crowbar in 2 seconds.