RE: Center of Gravity contraption
Just use your fingers.
People tend to get way too critical when it comes to balancing a plane. While it is true that balancing is vitally important, the exact spot is much less so.
You must have noticed that a manufacturer will not give you a CG "point", but rather a "range". Exactly where inside that range the plane "should" balance is completely up to YOU.
If you balance the plane exactly on the recommended CG it SHOULD fly, but will it fly the way YOU want it to? Maybe, maybe not. A good example of this is my dad and I. He prefers a plane that leans slightly toward the nose-heavy side, while I prefer the same plane to be slightly on the tail-heavy side. So there is no "Perfect" point.
Here's how I balance a plane:
With the plane in a ready-to-fly configuration (minus fuel) pick it up at the recommended CG and see if it sits level. If it does not, shift or add weight until it sits slightly nose down when picked up at the CG point. Now of course comes the usual question: How much is "Slightly" nose down?
Simple answer, only enough so that you can say it is NOT nose-up. In other words, you might look at a plane and say, "Is that exactly level?" Do you need to break out a level and check it? No. Just add or shift weight until you can look at it and say, "It is definitely not sitting level." At this point, you KNOW that it is NOT tail-heavy
The reason for this is because, as we all know, a plane that is nose heavy will still fly. It may not fly as well as you want it to, but it will fly. Whereas a tail-heavy plane can be, or quickly become uncontrollable.
So, now you know that your plane is flyable. Now, fly it.
How does it fly? Is it perfect? Great! leave it alone.
But maybe you notice that it's a little sluggish on the elevator response, or it won't slow down enough on landing. If this is that case, you probably want to move the CG back a little. So, do you break out the balancer? No! Just shift the battery back a few inches, or stick a 1/4oz of lead on the tail and fly it again.
Keep repeating the process until the plane flies the way YOU want it to. At this point, do you need to check the CG? No, why would you need to find out where the balance point is? The plane is balanced and flies well, so WHERE it balances is a moot point.
The only time you may care to check the balance is if you should decide to change an engine, or add a smoke system, or do anything else that will drastically alter the CG. In that case, once again, lift it with your fingers and mark the point where it balances. Now make whatever changes you want to make and re-balance it to the same place that it balanced before.
So you can see that there is really nothing critical about balancing. It's not like you only get one shot to get it right. As long as you start in a slightly nose-heavy condition, you can always do the fine tuning later.