OOAK = one of a kind, which means you can put anything and everything including the kitchen sink into them. In fact there are some sellers on Etsy who specialise in this kind of batt

It sort of turns left-overs into designer products! Seriously, its a good way to get an accent yarn with a little luxury, but I don't think they come cheap. They certainly sell quickly!
They are quite easy to make. You need your base tops or roving, divide it up (a spare coffee table is good for this, I have a large tray), and then start adding bits to it, from alpaca, or silk, to 6 inch lengths of that cheap nasty nylon yarn you got on special last year - it all goes in, and gets carded together. How many times depends on how you want the finished product to look. I also garnett any ends of homespun, and add bits of felt. Depending on what you put in it, depends on how classy it is - my recipe is a bit feral, but the top of the range batts use buffalo, cashmere, etc and the colours probably all blend together nicely without clashing.
Its great fun, and you can never be quite sure how its going to turn out, hehe!
My mountains are rather static at the moment; the youngest and newest fur baby is teething, so anything wood has been put out of the way - the end result is I don't know what to do with my hands as I watch TV,