Music: [oddmusic] DIY truss/bracing solutions? (for very low tension)

So I've been working on this sort of guitar/percussion/plank thing, which
is a smaller version of my Specto instrument (
http://www.rodrigoconstanzo.com/specto/), and now that I've finally got
strings on it, it's bowing ever so slightly.

Here's a picture of the instrument:

http://rodrigoconstanzo.com/specty/specty2.jpg

I'm using relatively thick plywood, so I thought it wouldn't be an issue,
but it turns out it is. It doesn't bow much, since it's around 18" scale
length and using 10s and 11s as strings, but I wouldn't want it to get
worse over time.

I have the wiring running along the back, so I can't just attack another
layer of wood across the whole of the body, same goes with flush with the
surface metal stuff.

I was thinking maybe something like a truss rod (adjustable, long threaded
bolt) that's hovering a few millimeters off the back, but thought that
might be overkill. In the past I tried using another thicker string, adding
tension the other way, but not sure that'd be any better a solution.

Basically I want to add some truss/bracing action but with minimal
parts/size.

Any thoughts/solutions?

--
http://www.rodrigoconstanzo.com

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