I looked up bakfeits on google images and saw some. I know there are some that use rectangular tubing for the platform area. Most commercial cargo bike builders are building at a high enough rate they can get tubing drawn to their specs. Most use round and other typical bike tubing shaped tubing where it's more appropriate
this is my favorite set of bikes
https://www.google.com/search?rlz=1C...2VVTdfgbzRn_M:
Of course, with a few exceptions, the Netherlands is pretty darn flat, so weight isn't as much of a concern