ok, I went to a gravel and decorative stone place, they didn't have any slate, but i found something else plately and decided to go with it.
This is most likely going to be the last tank re-arrange. It took me forever to split up the rocks and sand all the edges smooth. But i was rewarded with my betta immediately going into the cave (there are actually two, but one is not visible from the front).
what do you guys think, do i finally have enough hiding spots?