It's well worth making one, just find a suitable computer fan, cut off the blades and stick a hard drive magnet on it.
DIY stirrerIt doesn't have a hotplate which is arguably more useful, but you can arrange something like a bowl of water with a heater in it surrounding the jar or whatnot
While those are very quick to make and effective, they do make a lot of noise. For stir bars, a simple zinc plated screw can be used, it does react with lye during STB, but so extremely slowly it would be just foolish to worry about it.
Youtube for plenty of examples.
SWIM's experience has shown that you need to sometimes vary the height and especially the stirring bar/screw/object to get it started, but its really not that difficult, and SWIM recons it can decrease time and/or increase yields in the same amount of soaking time.
If you do choose to make one take some time to find the fan with the highest possible power rating (I would recommend 1.5W at the very very least). Higher power gives it enough torque to support bigger stirrer bars and physically stir more liquid.