These two haystacks were built some time ago. Unfortunately I do not have any work-in-progress shots to illustrate this post.
Photo Two - A rough sketch to show the general layout.
Photo Three - The smaller of the two stacks.
Photo Four - The larger one.
The two models are built from DAS modelling clay over a simple white foam former. The twigs are from the garden, so are the stones which were attached with elastic thread rather than string - this gives a tauter rope effect in this scale (coat the elastic thread with uPVA glue).
Artistically, most novices paint their haystacks bright yellow, while most are in fact grey. In gaming terms a brown/grey works best. Secondly, haystacks range in size - from a few feet tall, to house size structures. Try browsing Google images of haystacks (there are a lot of famous artists who have painted haystacks, these maybe of interest). The stones and twigs were to stop the haystack being blown away.
Modellers should also note that haystacks differ quite considerably, with very distinct regional shapes and sizes.