How about some very thin stainless steel wire, a guy I used to repair flexible endocopes for used to have it for surgical instrument repair. Just wind your own coil, a little bigger than the hole then as you insert it into the hole you rotate it such that the diameter reduces, when it's in rotate slightly back the other way to lock it in place and trim to length. They'd br like the flow disruptors show in the links BB2K posted earlier.
BillA - a question about the parallel drilled block. Isn't it preferable to not have the channels too narrow (or diameter) as it will promote and maintain a stronger laminar flow region. I seem to remember reading that on a site discussing laminar-vs-turbulent flow, possible from a link you posted somewhere. Isn't it the case that the boundary layer is a greater percentage of the tubes diameter in small diameter pipes and that is why laminar flow becomes more dominant as the diameter decreases?
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