LoserProvocateur

I'm trying to print propeller ducts (link below) where the outside tolerances are fairly lose, and a little bit of balled up TPU won't hurt anything. The inside tolerances however are critical. The closer the prop can approach the duct, the more efficient the system is. I assumed this would be as simple as setting 'avoid printed parts' to off... but it seems that cura interprets this to mean the void in the duct is free to travel through. Is there any way to block travel (and the associated stringing, and balling) through the voids in the ducts, and force travel to the outside of the part only? https://www.thingiverse.com/thing:3351513