How to speed up building at game?

I have a very annoying problem. My game is about 2D characters in a 3D world. But my problem is the design of the floor and walls. The 3D model is separate from the 2D floor drawing. In order to build the world I need to put each image step by step on the 3D model. Is there a solution to accelerate the building process and improve the quality ?

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If you have the image as a texture (or even just as an asset) you should be able to just drag and drop onto the walls.