For basic 3D models (think project boxes, adapters of one shape to another etc.), give OpenSCAD a try. Add "BOSL2" library into it if you need threads or some extra niceties.

It's not a CAD in the traditional sense, but since it's basically a scripting language for 3D primitives, it works surprisingly well in LLMs. And since it's just text, there's not even any MCP or agents necessary (though it can save some time).

Basic test : open a chatbot of your choice (I've tried Gemini 3.8), ask it for some basic box for your model of Arduino, or give it some napkin drawing with dimensions and ask it to give you an OpenSCAD code for it. Then paste to OpenSCAD, render, take a screenshot and mark on it what you don't like and prompt again. Usually it's pretty usable within 2-3 such iterations.