Rob Spangler
This is probably more than you ever wanted to know about the subject
http://industrialscenery.blogspot.com/2015/11/carrying-grain-in-boxcars.html
https://model-railroad-hobbyist.com/node/28945?page=1
https://prototopics.blogspot.com/2019/01/shipping-grain-by-box-car-and-grain.html
http://cs.trains.com/mrr/f/13/t/90694.aspx
O scale Food Service Boxcar with grain doors and roof hatches and prototype discussion and photos
https://ogrforum.ogaugerr.com/topic/sf-food-service-box-car
HO scale doors
http://greatlakesmodels.com/Product%20Detail%20Pages/HO%20Scale%20Product%20Detail%20Pages/020%20002%20810%20006%20HO%20Scale%20Boxcar%20Grain%20Panel%20Door.htm
HO grain boxcars
http://tycotrain.tripod.com/athearn-rollingstock-resource/id187.html
N scale boxcars with grain doors installed
https://www.pwrs.ca/announcements/view.php?ID=9121
Tons of photos
https://www.bing.com/images/search?q=Grain+Boxcars&FORM=IDINTS