That DIY HX method seems sound, but wouldn't solder break during the bending?
And, if you brazed, the joints might be so stiff they would stress the tubes.
Plus, the
area of contact between the two tubes is very small.
IMHO, if you put them side-by-side and wrapped (like you would tape) them
together using a roll of thin sheet metal (copper or aluminum),
the conductive 'tape' would really enhance the transfer rate.
The rubber insulation tape over the outside, would hold the conductive wrap
tightly to the tubes as they would being bent.
Some durable heat transfer compound under the foil might help also.
But, it should be some long-life stuff, that won't dry up and turn into insulation..
That Bucket-o-Foam idea is a good one.. I was thinking of building a box
out of blue foam insulation (that's what we use at work).
I would leak test for pin-holes before starting, and also before shooting the foam into the bucket..
Cheers,
Rich