Making the mask is easy... just cut out a mask shape from some vinyl material, and use it as a mask. Here are some tricks:
Not all vinyl is created equally. The adhesives vary. Stay away from low stick, or water soluble adhesives.
The positioning backers used to apply the vinyl (to help it hold it's shape) will leave a glue residue, and contaminate the surrounding surface... don't use it!
You'll want to experimant before trying this on any good stuff... here is the process:
1) Ano, and dye the part the graphic color.
2) Apply the mask.
3) Dip in 100% bleach, and rinse well.
4) Dip in 25% Sulfuric acid, and rinse well.
5) Repeat steps 3&4 until all trace of dye is removed (DANGER, do not mix bleach and acid, even rinse water... POISON).
6) Dye the surrounding color.
7) Seal normally.
Pull the mask, and remove glue residue with lacquer thinner.
9) Continue sealing for an additional 10min.
If bleach has seeped under thye edges of the mask (it will leave a silver ring), the vinyl was not the right stuff.
As always, avoid contaminating the raw ano... always use gloves, clean rags, and clean laquer thinner.
GL -Acid