what about taking a piece of say, .5mm to 1mm thick copper sheeting, cutting a strip, and wrapping it around a dowel the same diameter of your hole minus the thickness of your sheet (would have to sand down the dowel to the exact size).
you could control the width of the gaps between the copper, and then you could put the tube into the drilled hole. i doubt you'd have to secure it if it was snug enough. perhaps you could tin the outside and then heat up the block when the loop was inside, melting the solder?
again, more ideas :]
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