I created these little adapters to secure the three booster tubes in the Falcon 9 Heavy.   They are located where the pneumatic actuators would be on the real thing.   This is for the Boyce Aerospace builders kit, which relies on the thin booster conduits for the job.   They may work, but I didn't want to chance it.  They fit at the top portion of the booster (bottom held together quite strongly using another design). 

Printed on my MK4 using 0.20 / 40% / PETG.   I printed then sprayed with plastic adhesion promotor and then painted white.   I didn't want the heat of the day (or while sitting on the launch pad) to compromise the integrity.