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 limitation: https://bugs.mojang.com/browse/MC-54254
You'll be able to create custom shaped blocks when Mojang will add this feature: a special tag that can be turned on or off, now it's always on. "Opaque".
Another reason is that I can't change hitbox of blocks.. sadly another Minecraft limitation.