I purchased a 8+12 winder on ebay for about 130 shipped that did well for the time I used it. I ended up selling off quite a few of my watches and now just set them manually when I need to. It's kind of a pain when you have autos that are day/date as well and therefore 3 different things to set before wearing them.
To the OP : I looked into manufacturing my own winder, but never did it. You're going to need a low RPM motor that is a commonly available voltage, IE 12v, 9V, etc. The part that is more difficult is the circuit that times how long to turn it one direction, switch direction, and then shut off. I never ripped into the one I had to see how they put the circuit together, but there are many different ways to do it. Ideally you would want to also include a rheostat so you could control how fast it actually spun, that way you can tune each winder motor to the particular watch that is on it, as you want the least amount of turns per day as possible, while keeping it wound.
The other difficult part is creating the holding cup. If you angle it like most cheap winders, it's not really an issue, because gravity will hold it in. But if you make it vertical, like the nice orbita winders, then you need some sort of retention device. It could be as simple as a velcro strap across the front of the watch or as complex as spring loaded clips that pop out of the cup once the pillow is fully inserted.
Its up to you how professional you want it to look. When all is said and done though, I predicted not a huge difference vs a really nice orbita unless you buy parts in bulk and don't value your time too much.
