Buildable. 1x1. Cost: 20 (or 25 or whatever) straw, 10 string.
RIght click on the hay bale to extract a unit of straw (the same as the unit you put into it). When all straw extracted, hay bale goes away. A storable 20 or 25-unit space for your straw, with low cost (string is freakin everywhere). Keeps a listing of all the straw q's you put in and takes them out one by one, probably in any order whatever doesn't matter. Not liftable, but perfectly useful!
Something to do with one's straw to hold it instead of taking up other storage space - but only good for straw. Something one would see in an actual farm too (though normally baled with a baler, this is done by hand, thus can only hold 20 units, not very much straw at all compared to how much straw is in a real bale of hay.)
EDIT: On further thought, make it liftable - it now is preferrable to a stone mansion buildpost!