Finally I started sewing the chambers and the panels together, which is a good training for a sewing newbie (rather sewgasthenic). Double sided adhesive tape made it far easier.
My old Pfaff 437 industry sewing machine is rather for professionals, so I changed the original clutch motor to a servo motor, which enables stitch by stitch sewing. The machine has double transport, zickzack and a major advantage is, that one can spool the lower thread wile sewing, so I only need two spools and do not have to spool separately in advance.
As fabrice I used Paul's suggestion but a little heavier (220gr/m²), the thread is Serafil 30 and I use a 120 needle. After adjusting and oiling the machine everything works fine, no thread ruptures etc. The rolling tables are some helpful, particularly when sewing te panels together, but not ideal, they should have balls instead of rolls. For finally assembling the panels I will need a helping hand, thus it will be done in less time. Without help the 4 lower panels were sewed together, but I found that rather difficult. I will put together the 3 upper panels and then ask for assistance.
The result of my efforts will be seen once I am finished, there is an orphan 8m flagpole right opposite to my premises, where I will hoist the sail testwise, once it is finished;-)