All finished.
The "holder" on top is a piece of 1"x1"x1/8" aluminum
angle bracket, from Home Depot.
I chose aluminum over steel because
it's lighter,
softer (easier to work with) yet strong enough,
plus it looks shiny
.
The battery butts up against the lip of the tray on the driver's side.
On the passenger side,
I take a rubber stopper (also from Home Depot), drill
a hole through it, slide the stock battery retaining bolt through it,
and bolt it into its stock location.
I get a stopper that's slightly too big, and then trim it a bit
(with an utility knife; careful!)
for a snug fit.