The best $4 you can buy. Brass tube from Ace Hardware that goes into the butt of club. Yes they have the alignment sticks for this drill, but you have to hold it in your grip which interrupts your actual grip.
basically if you early cast, the tube will hit your body. Instant feedback. If you keep correct wedge and lag...the tube will not touch your torso :)
Creativity knows no bounds here in the SG Community!!
Thanks for sharing, @jon - let us know how that works for you!!