Kayfun V3 (might also work for V5) restrictive airflow hack.
My Kayfun V3 Mini is way too airy, even with the screw all the way in. From what I know, Kayfun V5 is even more airy, so not exactly perfect for MTL. You can buy a €10 "M2L" restriction kit, which is totally idiotic in my opinion since Kayfuns are supposed to be for MTL, so I don't understand why Kayfun V5 is so open.
But you can do a very simple hack, and this should also work with Kayfun5.
Disassemble it entirely and take out the center post (the thing on the picture).
Take a QTip or any other cotton swabs, those with a plastic stick. They are the perfect size!
Cut a tiny ring from the QTip, about 1-1.5mm (0.04-0.06inch) length.
With precision screwdrivers or similar (most vapers should have adequate tools), insert the QTip ring, from the bottom like shown in the picture. For V3, DON'T push it down, but just insert it barely so you can put the grub screw back in. Now screw in the airflow/grub screw, which will push the QTip ring forward as well.
Should also work with V5, although it might need some experimenting for the right position and size of the QTip ring.
The result is much more restricted airflow.
If you want the ring out again, push with a screwdriver or some tool from a vape set from the other side and push the ring out again.
(Why my KF V3 Mini doesn't shut the airflow *entirely* when I use the screw, I don't know. The screw seems either too short, or the thread inside the center post doesn't reach down far enough to entirely close the holes with the grub screw. Yours may be different.)