I am not an “experienced breeder” but did have the same problem with a neonate suri. I chose to use a pair of deer skin gloves from home depot. I think they were more for me psychologically than for the boa, but the familiar strong scent could be helping. I have a problem with flinching and the gloves seem to cure me. While one of the other posts recommended not touching on the head, I almost always do initially. If I slide out the rack and am greeted by a lot of open mouth hissing, I gentle touch the boa on the top of the head with my index finger, almost pushing the mouth closed; careful on the approach here. This usually ends or severely dampens the hissing. Then I rub the sides a little, which may produce more hissing, then another gentle touch/rub on the head. Then I pick her up and we are done with the aggressive behavior. It took a while to get where we are and I still get hissed at over fifty percent of the time, but we are getting there. At least I am not getting bit.
I started of by just sitting there until she quit hissing; five, ten minutes maybe. Then I put my hand in the cage. When she calms down I leave. Do this a couple of times a week. The behavior should get better. Let the boa bite you if it wants to. Eventually it will get tired and stop. Any jumping, flinching, etc will bring out the aggression. The more calm and assertive you are the better they are. I think the magic moment for me was realizing she was not evil, she was afraid. This allowed me to be patient. I never leave while she is aggressive. This is a pain in the butt, but seems to pay off. Be patient, going to fast will only stress your boa out. I never had any feeding problems with mine, so I assume she was okay during the process.
Again this is my experience with ONE boa. I have used the same tactics on another larger boa that is “moody” sometime and get the same results. Again, I mistook this for aggression initially, but now think the behavior is based out of fear and not aggression. Reading about handling on the larger python forums seems to support the principle I am sharing, but not the actually practice. Getting bit by these guys is a little more problematic.
Hang in there. Things will get better.
Al