What can really work well is an ambient background noise, such as the TNG engine ambient noise or whatever you are into (SWTOR station ambient, ...), wind, ocean.
The trick then is to put it on exactly when the focussed studying starts, and to turn it off abruptly when you (have to) interrupt, e. g. for phone, door, water.
That prevents you from the half-assed "I'm technically working right now" when you are really not.
Funny thing is that this is the ONE ADHD thing I don't have. My trick: Super-panic about being late.
The broader strategy is that I set an exact time that triggers the "panic mode". So for example when I need to take a 3 day trip, I put my open suitcase in the middle of the room and fill it only casually as is convenient, starting days before. Hours before departure, I'm putting what is still missing in, but very relaxed, and do other things such as shower, eat, whatever needs to be done. But like 20 minutes before departure, the "panic mode" is triggered. Whatever is still missing then is done with maximum stress, only absolute show-stoppers, no optionals, complete panic the whole time.
Knowing that panic mode is still there to help last minute, allows me to do the entire thing very relaxed.