Yes, I have made a welding fixture if you are thinking on it. Basically the chainstay tube positioning will be made on those two little shafts on the left side. Those are tight fitted geometry and preventing the tubes from warping if you slide all the way up them. It will be supported with also the welding fixture just to make sure the distance between rotation points will be perfect.

 But length of the chainstay don't will be positioned, so I think taht it would be good to use something to fix front and rear pivot points. If not, when You will weld top surface, You will have compresion on the lower part of profile. When you will start welding lower part of profill compresion will be relise and profile will get shorter. When welded parts are fixed, liquid metal crystalize matching to surfaces. When you will take your chainstay out from welding fixture contraction will be much smaller.

 @Bogumil1202247: Good advice, thank you mate!

 No problem
I'm glad that someone has got more determination than me, to transform own designs in reality.
By the way front triangle also need some welding fixture.
Accuracy of the frame is very important thing, because it has big influence for mounting tensions.
Of course some surfaces are enough flaccid and low accuracy isn't problem but it is issue of construction.
Of course no one will do super complicate welding fixture for prototype.
That's why many companies use simple method ad straightening after welding.
Some companies use welding fixture only for tack welds and welding process is without welding fixture.
Below I added some pieces from my master's thesis, maybe it would be useful:
www.pinkbike.com/photo/15749140/?s6
Also it is very important to finish welding in places where while riding are low tensions.

 @Bogumil1202247: Damn, let us continue the discussion via Facebook if you don't mind, because I've got lot to ask. My username is Zsida Tamás

 My nic on facebook is Bo Gdan