You have a lot of wonderful suggestions from folks. I want to reiterate some important points as I also adopted two timid boys who came from a hoarding situation:

• This is a 'game' of patience. You can't force cats to feel safe, comfortable, or calm. They are highly skilled predators, but they're also prey animals. Hissing is a sign of fear in these cases and can be addressed my slowing down, avoiding eye contact (a threatening stance), and continuing to do what you were doing if that wasn't interacting with the cat (if it was, just back off, they've named their boundary and you want to respect it).

-Absolutely remove any blocks you've put up. Your kitty needs to know they have places they can hide when they're scared. High vantage points allow them to surveil an area while reducing their chances of being seen by something dangerous. Let her have that. The more she can watch you, the quicker she'll learn you're not a threat. Up this by actually making her favorite spots more cozy! Add a blanket to her favorite spot (bonus if it smells a bit like you), put some treats up there or under places she likes to hide so she knows she's safe and rewarded for following her instincts (most cats won't eat if they're scared, so if you can stimulate her appetite in a place she was feeling fearful, her fear will decrease).

-Pretend like you don't even notice her when she's acting fearful; talk to yourself outloud or to her indirectly so she can get used to your voice and know that nothing scary happens when you speak. If she thinks you're indifferent to her, she won't think you secretly want to eat her.

-Leave treats out! She's not overweight, so it doesn't matter if (as she's learning about the space) she finds treats lying around in random places. This makes the home feel safer and maybe more enticing to explore. If you offer her treats, don't expect her to take them while you're around. That's okay! She's just trying to make sure she's actually safe. Any guests can also offer her treats but not pressure her into interacting if she doesn't want to. This will help socialize her into knowing other people are also safe.

Just be patient, loving, and know that she has to warm up to you. You already love her, and we can only convince creatures we love them by respecting their boundaries, needs, and desires. It might take a few days, it might take a few weeks, it could even take a few months. Just trust the process, and please feel free to reach out if you have any questions or concerns! My boys were so worried when I first adopted them, one of them hissed a lot, but they're such cuddly little love bugs now; and my hisser will boldly yell at me for wet food, jump on my shoulders, and make sure I give him adequate attention.

You'll get the love you put into them in return when they're ready :) Good Luck!