I'm building myself a stir plate out of some things from my scrap heap of computer parts (mainly an old external hard drive and a case fan). I've used it in its prototype stage for one yeast starter already. It worked well enough, but it was a bit finicky, especially once the starter got thick with yeast.
Since the initial testing I did with a flask of water didn't accurately represent the stir plate's performance on a yeast starter, I want to test it on something thicker as I put the finishing touches on the build. I probably should've saved a jar of trub from my last batch, but it's too late for that now.
How can I make a couple liters of something with the consistency of a yeast starter for cheaper than actually buying a pack of yeast and making a starter?