I see almost everyone running into this issue while building a rig with the Obsidian 800D case and a SSD drive.
The screws don't come with the case.
The screws don't come with the SSD either.
I bought the following screw that worked on the Samsung 256GB SSD.
The silver screw on the left is the one you need, I bought it from Fry's.
The black screw on the right is the standard HDD bay screw comes with the case and it doesn't work.
Below is the picture of the package for the screw that works with the SSD.
It's a tight fit and you can see the bottom of the screw from the side as it goes down so make sure it's not too tight. And yeah you don't need a flat head screw. With this ball head screw you have to push the drive down a little bit as you insert it into the bay and it fits perfectly. I'll try to post picture of the SSD with the screw in it.