I've designed a click pen mechanism specifically for 3D printing. It has 50mm diameter, the lenght is 138 mm, and it designed in a way to just move the mechanism about 25mm, it's a fundamental model so you can customize it according to your action. I checked and ensuring both thickness and strength. I don't recommend rescaling the model, as it may lead to dysfunctionality. Feel free to contact me if you need any assistance or have further inquiries.