Taps aren't difficult to make. Don't last as long or cut as well as commercial taps, but OK for specials.
First cut the required thread pitch in a rod. Silver Steel is best because it can be hardened and holds an edge, but mild-steel copes with softer materials for a short while.
Then cut 4 slots along the thread to create teeth: I normally mill them, but a file works too. Finish, by filing off the flank, non-cutting, side of the teeth to provide relief. Not shown in the CAD pic, but the rod should be chamfered to lead the teeth into the hole.
That's a basic tap. Can be improved, for example by cutting the thread into a mildly tapered rod. As this type of tap doesn't cut brilliantly, it might pay to make three with gradually deepening threads. (A first, second and third set.)
I guess the main difficulty is the job will be a bit fiddly and delicate, at least by my clumsy standards. M3.5 x 1.0 is a relatively coarse pitch for a small diameter rod.
Dave