This lens took a few tries to get right due to the surface shape and the limitations of the fillet tool, extruded cuts, and thickness tools. My workarounds include creating a 2nd body to create the "cut", then using a boolean to cut the main body. The thickness tool would not work at all so I used the scale tool on the draft workbench (shout out to JOKO for that tip). Once the surface cut was made with the first boolean operation it became quicker to use a combination of the PartDesign and Part workbenches to create a shape and then use boolean commands to join or cut the bodies.
This lens will be 3D printed for a 1968 Serro Scotty Gaucho and replacement lenses are not available.
FreeCAD File and STL available here --> https://grabcad.com/library/arrow-53-marker-lens-1
Edit, Forgot the about info:
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.4 (GitTag)
Build type: Release
Branch: releases/FreeCAD-0-18
Hash: 980bf9060e28555fecd9e3462f68ca74007b70f8
Python version: 3.6.6
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)