I keep the case unlocked. It would be nice to use TSA locks, but as far as I can tell, the only real reason to lock it is to keep it from being stolen and if someone is going to steal it, they are going to grab the whole thing and they'll have plenty of time to break the locks (or get access to TSA keys). There are plenty of latches on this case so I really doubt that it would come open accidentally. Of course anything is possible, but I've flown with it a few times and it's been fine.
These days, Calton probably has a TSA lock option. I have an SKB rolling case with TSA locks that I would use because it would be too easy for the latches to pop open on that case.
It looks like this case will work out well for the Series I with a buckwheat hull neck pillow underneath the body to raise it up to the right angle. Still, I think a Calton is in my future.