Thanks for the comments.
I was about to make one using PVC pipes or something similar, the idea was to nest one side inside the other however storing the blade was still a problem.
Somehow at the hardware store I found myself next to the aluminium strips and pipes so that is when the idea of using the al strip came up.
The only hard part was to drill a hole in the blade tip .
That also happened by chance.
My drill bit became red hot and so did the blade tip.. letting that cool down made that part of the blade softer
(I had no idea about annealing , it just happened)
The other accidental discovery was that the spine is the same size as the blade so I could (following on to a suggestion from Dryer at the Backpacking forum..) if I wanted use a lighter wood only blade and store a metal blade on that spine.
Something like this :

(obviously cut to fit…)
That 9" blade is a standard length so you could have two blades (if stiff enough) and just turn the handle upside down
Sven saw..
(I had to look that up..)
The two designs I had in mind were the H version by Rob Kelly
http://www.qiwiz.net/saws.html
and the Buck Saw by Steve Evans :
http://www.suluk46.com/RandD%20-%20RD32%20Ultralight%20Carbon%20Fiber%20Buck%20Saw.html
Having those two in mind I deliberately not researched any other just to come up with my own version.
As it often happens someone else has already done that ..
When I finished building it, I had a look at Google images under "hand saw" , found nothing there but just tried "compact saw" and the Sven came up.
This was the saw that made me do it :

The problem with that it was hard work and you need two hands .
Franco