make the cap. from the surface, not from the sketch or extrusion, keep data connection as short as possible!
i don't know what is up, the zebra on that surface is gorgeous, i can't detect any flaws that could grow into problems...and i looked!
so we have to resort to lying. tell FreeCAD you're going to the bathroom, and when it thinks you're gone, fusion to the extrusion and apply thickness.
i only got it to work with 1 mm. take it and run!
this model cannot be filleted on the front sharp break. this is a show stopper. anything that goes to production needs a realistic fillet, or it is just a graphic.
remember: sketcher splines would let you model the sharp plane, AND a very small rounding to vertical in one curve. this method makes a manufacturable object right off the bat.
remember2: this model is ultra vulnerable to topological bombs. i tried adding a construction line to one of the sketches, and the profiles turned into a criss cross mess. deleting the construction lines did NOT fix it. that model was toast, had to redo it from the earliest sketches. i am very used to this, but it is a very fine line to walk, and can be really frustrating. if you build a big model without being aware of this, you could end up getting very mad at FreeCAD.
the goddamn son of a @%#^%$#*%^$#@^%$$@%$^ solid still won't take a thickness!!!!!!