Not my idea, I have to put that down to Pat. But, his post is an old one that might have been missed. There is a link to his site on my blog where the inspiration for my how to came from.
Don't get me wrong, these are not historically accurate haystacks with the wood supports, etc. These are wargame haystacks: They are throw against the wall tough, as all war game terrain needs to be given the way it gets abused - unlike figures, which are (why I don't know) more kindly treated.
How to here:
https://olicanalad.blogspot.com/2019/02/haystacks.html