I've found an effective way to tension the poly-V belt that drives the alternator that I'm posting here as a search hadn't yielded anything in this regard.

It is basically a bolt and long nut that I put as pictured and then loosen the nut while holding the bolt head (another nut in my case for lack of a bolt) to prevent it from turning.
This will push the alternator up thus tensioning the belt.
Tighten the nut&bolt on the adjuster as well as the two pivoting bolts and nuts, remove the 'adjuster ram' and go for a test drive.
When happy with the tension (no squeal after a cold start when driving off and accelerating up to 2k rpm) put the airbox back.