There are two main types - blocks and models.
Blocks are geometric shapes that can be painted with a brush - you can only select them from the available set.
And then there are the models. They have a more complex shape and are unpainted. We can mod them.
To set a form for a block, you must have at least these parameters:
<property name="Shape" value="New"/>
<property name="Model" value="wedge_tip_stairs"/>
<property name="Texture" value="241"/>
And to set the model, these:
<property name="Shape" value="ModelEntity"/>
<property name="Model" value="#Entities/LootContainers?sports_bag01Prefab.prefab"/>
Dart trap - is a model, because its shape can not be changed as well as the block.
Here you need to either change the entire model, or change its type to block.
But this not work without the original model, there additional data about the muzzle-location and electrical plug.
To create such models in Unity, you need to know some rules and location tags. I don't remember anyone posting tutorials for electric blocks, but I'm sure that someone who is well versed in Unity can do it.