Bastian,
I think the short ones are OK as they have appr the same lenght as the ones for the curtain.
My tubes are as the drawing states.
Connections are not the heavy type, but similar to 12x12 ones -> they have a different shape (and maybe angle).
I don't have a blackout curtain, but I can imagine that the fabric will be similar to the black out curtains in communication bodies eg.
My frame doesn't have the "metal wire" crosses. Once on a show, I saw a TA unit with this shelter, not using the wires. The picture is somewhere on this forum.
Hope this helps.
Piet