Hi Niko -

> Even with two rails this particular set-up (my window frame) produces
> openings at the corners.

I think that's because your 2 rails are not set in proportion to one another - if you look in the Right-side view:



Note the different thicknesses between the rails indicated in red.

If you check out the file I attached in the previous message, if you look in the side view you can see that the rails I put in there have an even thickness between them throughout, so the shape does not warp or scale and the pieces match up.

- Michael