I have a semi-diy hotend setup on my MOOZ-2 (an obscure chinese printer). It requires that the total nozzle length (including threading) is longer than 15 mm. So far, I've been using Volcano nozzles because they're the only ones I can find that meet this requirement. However I believe this negatively impacts performance because they stick out way beyond the heater block (see picture below). Unfortunately, the next size I can find are MK8 or V6 nozzles which are 13 mm - too short! Are there any nozzle sizes in between that I could use?
Yes there are, the vendor of the printer sells 17 mm length spare nozzles for reasonable prices.
Could you get an extender made? That would give a larger metal body to hold and conduct heat to the nozzzle tip better, and would allow you to use standard nozzles. What I'm thinking is a cylinder of around 9-12 mm diameter, with one end machined then tapped to be M6 external thread, and the other tapped as M6 internal thread, with a 2 mm hole through the center. If you know someone who has or has access to a lathe and taps, they should be able to do it for you.