If you want to try to get a good tambora, here is a great way to practice it:
1. Instead of plucking the strings, hit the strings with the side of your thumb. The entire arm may be used. Make sure that your hand bounces off of the string. You don't want to mute the strings. This is similar to hitting a snare drum, and letting the "note" ring.
2. Hit the bridge in exactly the same manner. Make it sound as full, well-rounded, and musical as possible.
3. Finally, with your ears, search for that sweet spot somewhere in between 1. and 2. yet far closer to 2. You are trying to get the best of both worlds. You want to hear the notes from the strings, but you also want to hear the percussive thud from the bridge. At this point, year ear may be far more reliable than visual reference. The tiniest of hand movement will significantly alter the sound.
Dr. Todd Tipton, Noda Guitar Studio
Charlotte, NC, USA (available via Skype)