Is there enough room inside a MacBook Pro 16" to fit a slim Tile?
No, you can’t fit them.  I have a 2014 MacBook Air and my wife’s friend has those tile key dongles. I folded a piece of paper until it was about the same thickness.
I opened the back and tried to find a place to fit it. Even by the battery where I thought there would be room made the back cover not fit. I was using paper which is more flexible than the plastic tile and I was afraid it would break something. I would definitely not do this
If you want to still use the tile, maybe you could hot glue it to the outside. I like hot glue here because it will stick good and when you want to remove it, it won’t hurt the Mac. I know this because my daughter hot glued rhinestones to her MabBook lid and it didn’t do any damage.