Grassi, to stick your needles in place I'd suggest epoxy resin (eg Araldite, if that exists over there). Put it on the needle rather than in the hole if you don’t want to have to clear them out afterwards…
Silicone isn’t that much of an adhesive - especially if it's applied thinly - so it probably wouldn’t be a good idea.
You could also try superglue (cyanoacrylate ?), but it does dry rather quickly
As for using more tube for the cups, I'm not sure that it's a good idea because:
- I don’t know if you can solder stainless steel?
- You'd have to make the holes further apart
- As said above, stainless steel is more of an insulator than a conductor of heat.
- Drilling 1.8 to 2mm diameter holes in copper is not a problem anyway
