You can't create solid custom slabs, stairs and solid placeable items other than blocks.
Problem is that Minecraft would show them as transparent blocks because of this bug: https://bugs.mojang.com/browse/MC-54254
You'll be able to create custom shaped blocks when Mojang will fix this bug (adding a special tag that can be turned on or off, now it's always on. "Opaque").
Example that shows this Minecraft bug (2014 screenshot, I hope they'll fix...)
