re: training aid
I think I tried numerous gadgets and training devices. The inside approach isn't bad. It's designed if you swing "over the top" or have a problem swing inisde out. But a word of advice is to save you some money and build it yourself. Do a good search for a home made version. It cost me $5-10 to build. All parts can be found at your local Home Depot or Lowes.
Another version that works extremely efficient is a simple "shoe box". Take a shoe box and put in about 1" outside of your ball target line. In other word, give your club just enough room to clear. Also make sure the box is a little more in back of your stance. Now, if you have an over the top move, you will hit the box, because you crossed to the outside of the target line.
This was one of my first drills at my first lesson. I can't tell you how many time beat up that shoe box. However, over time you will get better.
That's of course if you have an over the top move, which most amateurs do when they first start out.
Good Luck.