When wiring a new 2 story house I will sometime leave extra romex feeds,RG6 and phone line in the attic.I always nail the boxs to a roof rafter so I don't have to go looking for the box at a later date.
Wouldn't nailing your J box to the rafter solve your problem.
BTW-Leaving those extra feeds in the attic have made me look like a hero more than once.
