For the fencing, what the companies in the US use is a post with feet that the fence panels attach to. It's shaped more or less like an upside-down T (or an X on the ground with a post sticking straight up from the crosspoint), though sometimes there are small diagonal bracing struts from the post to the feet. With the existing fence material design (which I really like, BTW!), you might need to make the vertical post more like the H-beams in the buildings (and I can't say I like that idea much more than you do, building H-beams out of MDF must be a pain in the butt!). Maybe raiding some Plastruct or Evergreen Styrene architectural pieces instead of MDF?