I've stenciled on top of Zim twice. I just cut out the numbers from a decal sheet, and taped them together. Use an airbrush with very low pressure, and mix your paint very thick. I've had good results with Yellow.

Tank zone sells a stencil template, but have been out of stock for some time. It isn't cheap.

I've also tried hand painting the numbers on, but it is very difficult to get a uniform line on the zimmerit. I usually make a stencil for the balkenkreutz and then add the white outline by hand.

Good luck.