Transparent/custom shaped blocks
Currently is not possible to create transparent blocks or custom shaped blocks. This is because of a Minecraft limitation as you can read there: https://bugs.mojang.com/browse/MC-54254
I order to make this possible (before Mojang will fix this issue) you can create a furniture (but won't be solid obviously) which is an entity and not a real block.
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...)
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