Actually I did think of this. They're called flush mount hangers
and I have a couple that look like this

but bigger (about 2.5") from ACE hardware.
Two things I noticed from using some on a shelf in my son's room:
1) When locking the pieces together, you have to be RIGHT on,
it's not THAT easy to put together (a VERY snug fit)
2) The screws that hold each separate piece to wood are
pretty tiny, maybe only 1/4 to 1/2" ? I'm worried that even
if it's strong enough to hold the modules, it may tear on play.
Hence my brainstorm on the bolt/wing-nut idea. Unclet has
some good points though, it's making me waffle back to maybe
the Autobot design, maybe a bit deeper to get better visibility.
I'm having a buddy over tomorrow to help me with spacing, will
get a better idea then.