Years ago coal was delivered to many homes and businesses. A lot of times there was a metal door the coal delivery company would open and pour the coal through. Well, I was on facebook and someone asked in a model railroading group if anyone made a coal door as a detail part. I asked him what they looked like and what size they were, and not too much longer my Anycubic Photon was churning out the part.
It wasn’t a complicated model. I designed it in sketchup, then scaled it, and sliced it in chitubox. A sped up screensave of the design process can be found on youtube.
The coal door can be downloaded from thingiverse