Made some more progress in the last couple of days.
This is how I backfilled the Ion Arm. 3 full tubes of plastic bonding epoxy. This actually hissed loudly and softened the plastic with the extreme heat from the reaction. DO THIS IN STAGES or use a longer-setting (60m+) epoxy. This wasn't my best idea.
Hand cut speaker grille using a 3D printed template.
3D Printed cable clamps. Works great!
3D Printed 2nd booster tube>cyclotron connector. The booster end is just 1/4" loom glued in place. This has to be smaller (10mm instead of 14mm) because there isn't enough room between it and the motherboard.
Electronics in the pack shell, along with the flashlight reflectors used in the Cyclotron.
Electronics on the motherboard, speaker, brackets, etc.
Basically done at this point. I've since weathered it and added the missing R701 label on the pack, as well as filled in all the shell holes and original backpack strap holes. Pretty easy, too. Cover the area with blue painters tape formed to fit the detail, then backfill with a good high-temp (Easy-flowing) HMA (hot melt adhesive). You can peel the tape away when done, and it'll leave a nicely finished, flat fill behind. Just paint to match.
All I'm waiting on now is my V-Hook from CPU64.