Similar issues here. I have been able to hover at 63% with 4 small cups of distilled water in my 75 count. I have put weatherstripping along inside of lid to improve seal.
What I am considering next is either:
a) putting humidor in my newly set up coolidor
b) putting humidor in one of those large "clothing-storage" ziplock bags. Seen these before? They are at almost any department/grocery store. They come in sizes large enough to stuff a body inside
With the ziplock idea, then you can throw some of that wetted down cat-litter crystal stuff I have been hearing about (AKA poorboys beads) inside the ziplock, outside the humidor and then the humidor will be surrounded by a 70% RH environment, which should help it keep any RH inside you like.
Cheers
Cyanide