Drowning inside the shell is usually caused by too high of humidity during the main portion of incubation--not right at the end. It causes the air cell to not develop properly, and there's not enough breathing room inside the shell. At this point, high humidity is a good thing, but obviously you don't want any wells or other places they can drown AFTER they hatch.
Good luck. You're doing great to get them this far with that set-up. Some people have hatched eggs inside their clothes (human body temp is very close to incubation temp--and a sweaty body provides plenty of humidity, lol), and under other odd circumstances. Of course, it's hard work and not as reliable, but I'm keeping my fingers crossed for yours. Good luck!!