I don't know if somebody came up with this idea before, but I thought I would share it with you.
My local DIY store sell these clamps for 12 mm water conduits.
They make cheap and stable mounts with proper thermal conduction.
On the underside there is a M6 hole, but you can mount them from the inside out with a M5 bolt too.
Beam height is 19mm.
I'm not sure if there are easily available in the imperial parts of the world, though.