Blockly is a library for developers who want to make drag-and-drop blocks environments. We don't currently offer a "level builder" or any no-code tools that would allow you to create a level like this without creating your own Blockly tool from scratch.
You can see the
source code for blockly-games on GitHub if you want to try modifying the level to meet your needs. However, generally since the Blockly team does not own or maintain Blockly Games, we cannot assist you with the internals of how Blockly Games works, though we can answer questions directly related to the Blockly library.
Best,
Maribeth