They are actually worthless with fresh straw!!!Banks need to be solid to help prevent your horse rolling over!!!I used to build my banks to about 2ft and leave them,mucked out the middle and took crap off the banks every day,they were about a foot wide!(This was with a gelding though,dont think i would do it with a mare as they pee in the banks and the smell of urine would be unbearable!!)Worked a treat but a git when you have to dig em out in the spring,lol!!No they never smelt as i never disturbed them(he got cast once and split open fetlock)!!You do need a good sized box!!
Rubber matting, or horizontal wooden slats are the best thing to help your horse get a purchase if he gets cast. The only good a big thick bank will do is stop him lying too close to the wall in the first place, maybe.
i have quite big banks about 2ft and 1ft wide, to keep her away from the walls & prevent draughts, she also lays against them and uses them as pillows. shes got cast before so i always bank her up - getting them solid enough is my problem as her bedding is always so clean it has no grip