We have similar feeders square for 1 bale and some longer ones for 4 bales a couple 2s and a couple l bale ones we built last. we use sucker rod in a a shaped pattern narrow on bottom never had a cow get their head stuck .From my dads experiment with slant bars 50 years ago the cattle waste less hay with the a pattern bars  2 3/8 oil fld tubing bottom middle and top bars suckor rods on silo rods  for the a s  and filler bars. Our oldest feeder are going on 50 years old .I have replaced a few bottom runners where they set in manure if they where made of pipe i instead of tubing, old boards in the the bottom skirts and we have made them taller over all height is close to 5ft now to stop the bigger cows from eating over the top . I guess my dads ideas where heavy and cheap  he bought several  feeders over the years and they all are in the junk and the homemade one are still going strong we pick .them up up and move them with a loader. Honestly  the one and two bale feeder are the most practical since you do not have  to have such a big loader to move them. Do not save much pipe with the 4 bale feeders ,I have 4of them I built.  I have almost used up my junk boards and had never thought of the plastic for skirts before  Usually weld several,of the sucker rods  every year but no big thing.