Good stuff. Doing that also for telescopes and binoculars. I would add to the equation the possibility to utilize factory made macro extension rings/sets. Which are dirt cheap off eBay. They will provide a better “skeleton” for you construction than any plastic could. Then just add mating parts going in and around that.
@Alex_Koukarine That’s actually what I did for the 400mm lens 
the 3D printed part is the black bit with the bolts sticking out the sides, and it’s actually strong enough to hold the entire weight of that big lens hanging off the printed part alone. The smaller lenses are all less than 50g each, so it’s not an issue with them.