Finally, 50/100 chests with detail. They can be open or closed (there should be a healthy balance of both here).
USE REFERENCE! PureRef is a great free cross-platform tool that will let you compile many different reference images into a single board, which you can then save and take around with you as a single file. Reference is extremely important here because your visual library is going to be considerably blanker than you think, especially on this topic. By learning to use reference images to inform your decisions, you will gradually commit different kinds of details and decoration to your imagination, which can then be used later. Observe things carefully - there's always going to be a lot more going on in these images than you think.
Chests can come in all shapes and sizes, with all kinds of little flairs. Some are banded with iron, some are rusted, some have additional planks of wood, some have filigree, handles, different kinds of locks.. The possibilities are endless. This is your chance to really push yourself to find new things.
Just remember - you will make mistakes. You will end up with MANY failed designs. That's no reason to be timid - be bold, all the time. Try new things, experiment, and when things go wrong, accept it and keep at it. This challenge is not about creating a hundred beautiful pieces of art. This is about producing a hundred pieces of crap.