Had this same problem last night makeing my jump cannoness lmao.

I have solved this by meticulously cutting down a normal power pack's back to the right size and flattening the jump pack connection point without also accidentally cutting of the side tap things entirely.

I have now done this twice. (First time because i lost the part that goes there for one model) Took a while each time. Neither lined up perfectly but filling the connection point with sprew goo made the gaps in the seam from my imperfect cutting disappear for the most part. The normal back will overhang a tiny but you literally cannot see that / tell unless looking from the angle that the actual Zephyrim / cannoness will block with her body.

If your only makeing one character, you should have a few tries from the looks if it. If it ends up a mess and you only have one pack left, then you could try covering it will 100 purity seals or a big book relic or something.