NPE "the secret in getting the acoustic sound out of the box/tub itself"

Well actually the two secrets... no, wait...

Among the many secrets .... :-)

But it is still a stroke of Genius to redesign the 'instrument' in that manner.

And it no longer works 'in the original way' as you have cleverly changed the underlying design principles of a 'cheap and nasty folk instrument' ... :-)

By mounting the bottom of the string in such a fashion as you describe, you cause all four orthagonal faces (and it will work the same way if you do the same trick with a circular wash tub, but the physics of a 'throbbing coinoid' are slightly different, but close enough for it work adequately in the same fashion) to pulse in the same outward/inward direction simultaneously. This tends to reinforce the outgoing energy pulse into a single 'circular' outwardly radiating one, thereby reducing the negative effects that happen with 'end around effect' robbing outgoing energy.

Very clever indeed.

Robin