Do a search on here for "Vanessa." You'll find a CG rig that is at least as accurate as the one you showed and which you can make out of stuff you've probably got and can buy cheap if you don't. On small planes, balancing on your finger tips works just fine and is even cheaper. You can measure throws accurately enough for any trainer or sport model with a ruler. Measure at the widest point of the deflected surface.

As for changing the CG, the ideal way is to move things like batteries to get the balance point where you need it. If that doesn't work, you'll need to add weight (usually up front). You can buy "stick on" lead weights which don't stick all that well.