Just a WAG here, but how about using a shaped wooden block for a
backer? I've done similar things for routing thin wood and the like.
You might even want to use a trim router and a template. We used to
use routers for cutting aluminum/foam/wood stringer/panel sandwiches
for putting windows in motor homes, so routing even aluminum panels
with wood-working tools is possible. Hot melt glue is a perennial
favorite for temporary fixturing in the woodworking world, might be
that would help, too.

Stan