I do NOT recommend any type of hay feeder that raises the hay above chest level, as it is unnatural for them to stretch up to eat. I do not like nets due to the potential of a horse getting caught up in them. you can buy or build things that are sturdier (a trough with metal bars to keep them from pulling whole flakes out)
also-My mare gets about the same amount of hay you are giving your two horses, and she is not working right now so that amount worries me. as well-i never feed formulated grain-i feed hay, oats for calories, and a pelleted smartpak multivite for nutrients. the pelleted feeds have crazy stuff like shoe leather as fillers and also usually dont have enough vitamins unless you are feeding them like pounds a day (which ends up being too many calories for most horses)
as far as boredom is concerned, there are many things you can do about that. working them more is number one. giving them toys is another…an empty plastic milk jug is my mare’s fave toy-she loves batting it around and crushing it and the crinkly sound it makes. i use it in her stall-tied to the wall with a single length of bailing twine-but since your horses dont have stalls…you could prob just toss them out in the pasture for them…things like human exercise balls and soccer balls can often be fun toys…then there are toys MADE for horses like jolly balls.
hope you got that shelter built for your babies with this horrible winter!