Thanks, yeah the solid bar can be cut to the required length, then drilled, filed if need be, always good to have some 12mm-20mm cuttings handy.
Adding extra mass just gives you a little more time before saturation, the heat still needs to dumped to the external air or your hand etc. Not too much difference in the smaller lights but it always helps a bit I believe and better for the LED (s). Better than just having empty space in my opinion.
Rather than a sandwich, a roll might be better with a solid copper core surrounded by a tight fitting alu tube.
Here’s one with copper & brass.
Or aluminium & brass
