Hello wouter
They have to be welded. Screwing doesn't make sense because it doesn't bring stability in the long run. If you want to do this, you should also weld the reinforcement plates on the A and B pillars.
To your other question:
Build 2 lids and screw them with a suitable gasket.
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Greetings Gerold
STi powered Typ4 in the works...
R.I.P. 1302/'72
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