I know when i bought my Aviator harness (just recently), it came with a DVD. They said on the dvd to get familiar with it first, away from the bird, to make putting on quicker and less 'fussy'. And to not practice indoors but wait until a day that you can actually do something special. Go outside, make it fun, etc, so that it is related to good times and 'liberation' instead of restriction. I find it must be working with Gilbert because he is becoming less and less fussy when i put it on him and like he is 'ready' to hit the yard with me. He enjoys the adventures.
Not sure if you tried that. Maybe even back off it and show Poppy the harness, give treats, make a positive association. I think if they make a negative association, they will be harder and fussier.
The dvd with the Aviator also said to do practice moves, like swiping your hand gently over the face to make the bird close his eyes, and holding up the wings for a count of three, etc. I did these things with Gilbert a few times before trying the harness and he actually seemed to improve a lot and 'get it'. I thought it was good advice.
Maybe this will help.