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A drafting triangle that offers some transparency.

Usually drafting triangles are clear so you can see your work below it, but that's difficult to do with 3D printing. 

Instead you can turn off the top and bottom layers in your slicer and modify the perimeter and infill values to get the desired stiffness and transparency.

200mm on the longest edge, but fully scaleable in X and Y (should be the same though…).

2mm Thick. For a stiffer triangle you can also scale in Z.