A couple of things about Cardinal when thinking about his armor:
"CD-0922 was one of the first of the new stormtroopers of what would later become the First Order"
"From the time I first put on this armor, he trusted me to keep him safe. He designed it himself because he said red was a color of power "
"Cardinal was presented by Hux with a personalized set of red Stormtrooper armor and given the name Cardinal"
So it was personalised armor designed by Hux early on in the rise of the First Order, could this explain where Phasma found a prototype helmet? But what other pieces did she use and what other pieces on Cardinal's armor was different to the standard FOTK's?
From the novel Phasma replicated a standard FOTK's set of armor and a prototype helmet but no mention of any other pieces so it really doesn't explain why she is so different to other FOTK's, did she design some pieces herself, did she copy what she had seen on Cardinal's armor?.
A lot of this build is going to be an interpretation of what could of been worn until more solid references can be found / produced.
Keep up the great work, hopefully more images will come available, there is still more of the Cardinal's story to be told and hopefully it will be.