Rather then cutting slots in the base plate by hand (which would have been too inaccurate to serve any purpose) we decided to attach the fins via a small tab. To accomplish this, a 3mm tab (90 degrees to the rest of the surface) was formed on the 50mm side of each fin to facilitate attachment. One attempt was made to affix the blades with a copper-based solder. The second attempt was not successful, so a combination of rivets and tin-solder became the procedure of choice for the subsequent 13 fins.

Why rivets and solder? Well, we found that the rivets held the fins in place while the adjacent fins were being soldered (and the entire plate had reached flow temp...). They also served to introduce more copper into the thermal transition region of the base plate and fin. The pins also added a bit of mechanical stability to the joint, and helped to eliminate any gaps between the two mating surfaces by the tension they enacted. Granted, the entire joint is not as good as a direct copper-to-copper swaged contact would be, but it's at least better then having a copper-air-copper contact because of really poor technique.