The two things needed to produce a very tightly packed spun hair body are: 1) never use a bunch of hair larger than your little finger (and one that is a little larger than a pencil or pen in diameter is best); and 2) use a hair stacking tool (I prefer and use the Brassie tool; but any of the hair packing tools on the market will work-in fact, I way back in the 1960's when I learned how to spin and stack deer hair, I was taught to use a Bic pen barrel and they still work very well) to push the hair back against itself after each bunch is tied on. When you puch the hair back against itself, remember to hold on to the hook with your other hand or it could bend. Also, after each hair bunch is tied in, take 2 turns of thread directly in front of it. And never put cement on until after a bunch of hair is pushed back on itself.
Doing these simple things will result in spun hair bodies that will be as tight as those you saw in the store. And you will also find out why spun hair bodied flies cost what they do because of the time it takes to do it properly.