Focusing mask?

[TheThing]

Somewhere, recently, I saw a link to a site that gave details of making a focusing mask with just two 'D' shaped holes in it - which didn't ned to be especially equal or accurate.

I cannot recall what the mask was called or find the website now.

Can anyone help at all?

[Bizibilder]

Is this the sort of thing:

Secret of the Hartmann Mask... Revealed! - IceInSpace ?

[TheThing]

No, it was two 'D' shape holes cute very roughly and crudely into card. The suggestion was that it didn't matter what size or how accurate they were.