There’s not much you can do to keep it from getting deep into your locks. As bleach processes, it foams up and expands. This is why they say to leave a gap between where you apply the bleach on your hair and your scalp as it will puff up a little bit as it processes, for example.
So, instead of worrying about keeping it from getting in, focus on getting it out!
As I learned in my geography class, folks like to say “the solution to pollution is dilution.”
If you can dilute that bleach (aka “pollution” if you will :P) as you’re trying to remove it, that will help it to stop damaging (or “polluting” if you’re still following my metaphor) your hair.
Hop in the shower and just let the water soak your hair as much as possible. Palm roll the water into your locks. Keep that hair soaking wet and drenched as you rinse, and rinse for 15-20 minutes (yes, I am serious, literally 15-20 minutes- make a playlist that lasts about that long or set a timer). However, your 30 minutes was pretty dang good, too!
Also, you can shampoo several times while rinsing, and you can also condition once or twice while rinsing, which is especially allowable since you’ve just done something damaging/drying to your hair and it will need the additional hydration and closing of the cuticle that conditioner can provide.
Rinse and scrub and it shouldn’t be a problem!
It’s a pain in the butt, but as long as you get most of it out and you strongly dilute it by rinsing for an ENTIRE 15-20 minutes literally, then it shouldn’t be able to continue damaging your hair or reacting.
Another idea if you are really worried would be to apply it with a brush to just the outsides of your locks. This may not give you fully even coverage, but it would only apply a very small amount of bleach that would be less likely to penetrate the lock as deeply.