I just received my 8400 yesterday and started building the frame.
The (very detailed) manual suggests beginning to assemble the whole frame with plastic corners but without fastening them. Well it didn’t luck so well for me and I soon had the all thing falling apart before I had a chance to put the first bolt!
So I rebuild it fastening the corners one by one with loosely tightened bolts and nuts and it went like a charm.
[quote=“raby”]
So I rebuild it fastening the corners one by one with loosely tightened bolts and nuts and it went like a charm.[/quote]
I think that is exactly what they had in mind. 
I had some problem , the first bolts it tells you to put in are the cable supports top real left and right
As the frame needs to be no gaps I found that tapeing it tight with masking tape worked really well and made it easy to get rest of supports in with no gaps and only 2 hands and made it very str8 and square when done
I had the same problem keeping everything together while assembling - used a roll of painters (paper) tape to keep parts together and it worked fine
I used tape on mine as well.
I found a video of someone putting his together and he used a strap.
Seemed to work well.