Here is the discussion
http://www.astronomy.ru/forum/index.php/topic,10421.msg244210.html#msg244210
The thread is long and there are several different masks. The one I showed in the first post seems to work the best.
But a few people speak Russian, so If you have questions, do not hesitate to ask. If my descriprion of how to make the mask for specific scope is not clear, please tell, I'll try to rephrase it