This is all good stuff, particularly the photos of existing slotted caps. I made a cut in some brass with a standard hacksaw blade, and although I couldn't measure it accurately, it's a bit less than 1mm, so would probably work. A Junior hacksaw blade is about 0.6mm, so will probably be a bit thin. As the slot will have an effective open area somewhat larger than an equivalent (vertical) row of holes, I'm wondering whether using Gary's existing 2 x 2 mm template would work - glue it on but cut down every second row, which would give 4mm spacing. Not sure how far to cut down, maybe three holes deep? Wouldn't be too hard using a fine-tooth hacksaw blade. The cap could then be soldered on, and if needed the slots could be extended into the cap slightly afterwards. However, Gary's idea of using a lathe is an interesting one. Fit the cutoff wheel into the chuck and make some sort of a clamp so the compound rest could be used to feed the tube into the cutter. At least I'd stop breaking drill bits!!