Take a sheet of shrinkable  plastic( size A5 i.e. 14,8X21cm ) and  draw a mask with  all surface felt-tip pens. Its size will reduce 6 times when heated. Bore a hole with a drilling machine.

Heat it in an oven for  a few minutes. Control the retraction of the plastic : as soon as it is shrinked,  put it out of the oven and flatten it well.