Please don't block up the hidey holes. If you do that it will take your forever to gain her trust. One thing that might help is look at the world from their point of view. Outside, where they are used to being, momcat could easily carry them away where she could easily get away from prey, or bad humans. They could go anywhere, because usually a feral is smart enough to hide in an area where there are escape routes and not someplace where they will be trapped easily. They had mom and siblings, and life was hard but it was good. They were learning to get along. Mom was teaching them to hunt and good potty manners.
Then one day, enters a pair of human feet. Clompy shoes, loud noises- (even if trappers whisper, cats have acute hearing). A trap was probably set down, tempting food put inside of it- (such as what you are feeding them now triggers memories). Mom gets trapped (I am guessing here) first. She goes ballistic, starts screaming, wets herself, freaks her kitties out. Kitties are grabbed, stuffed into a carrier than a moving vehicle, taken away with mom. Pretty soon mom is gone- siblings are in other cages and the family and world has been shattered.
Now from her hiding place, she sees the same kind of "feet" (again her perspective) she smells the same type of food, she is not in a cage in your way of thinking, but in hers she is. She is trapped and she is panicked. She has no way of knowing if this is a "safe" place or not. YOU know it is, and your friends here know it is, but this kitty has had her world rocked now several times and she will not appreciate it if you block off her hidey holes.
I always tell the people who I help to reset your timer inside. You are thinking wonderful lap kitty thoughts and that may very well happen, but it will not happen overnight. The minute you stop expecting her to behave in the way that "normal domesticated kitties" behave, is the day you truly understand that she is a feral. If you work on her timetable, and not yours, and don't expect anything out of her except to eat the food and use the litter boxes (you should have three boxes for these babies) then that is when the magic starts to happen and she will start to relax and come around more often.
On the catsite's homepage under care is an article called Handling Feral Cats. Maybe, if you haven't found it already you could read it and try some of the suggestions there.
Good luck with both of them. I have eleven ferals right now, some come in the house, but they all pretty much scatter when something scares them (like the other day, the sparkletts cooler gurgled and several of my cats sleeping on the window ledge leaped straight up about 5 feet and bolted out the cat door!
It is a feral response, they have two options- fight what scares them or flight. Usually the only ones who stay to fight are the ones who were neutered to late in life, or have not been neutered at all. Most of them will choose to flee- it is a safer response for them.
One of our members- Air Princess has two cats that she got when they were fairly wild. To this day, if something scares them, they run away. AP loves those two babies and she does nothing but nice kind wonderful things for them. But they still flee- it is their defense.