So, I think I may have the perfect solution on a portal chest that could work in expert mode.
First things first, they would spawn in caves where trolls usually lie. Their spawn rate would be 30%, so 1 in 70 cave trolls until you would get one.
Then when you find a cave troll that has one it would be a disabled portal chest which will have a grey color to it.
To Reactivate a portal chest you would need 10 diamonds and 100 time crystals, then you would have a portal chest.
This portal chest will have its own unique data thing, so it will not work with transferring from vanilla to expert, just expert to expert.
Likely hood of seeing this be added is I would be guessing 1 in 100,000.