Clearly, the creator of this hasn’t put together enough IKEA stuff. Half of those dowel holes should be the screw in thingies that mate with the 1/4-turn cam things on the other side. The holes on the top of the hand would be on the bottom, with the cams. Also, it would not say “disposable thumbs” on the page - It would give the product-line name on the front cover.

I’ve used them, and they are really handy.

Pull off your fingers, remove them from their fleshy prisons, let them be free