Parts required.

3/8 BSP male to male adapter or similar.
Inside diameter 5mm or less. Cut off one end and tap inside hole with 6mm tap.

1 M6 bolt around 30-40mm long and a M6 nut.

PTFE thread tape.

The above image shows the 3/8 BSP fitting with half cut off and the hole tapped out to 6mm.
The tensioner housing and the M6 bolt with locknut.

Notice the oil hole in the 2nd picture. Make sure the 3/8 BSP adapter does not go in too far and block this hole.
Use PTFE on the adapter and the M6 bolt to prevent any oil leaks.

When starting for first time only screw the M6 adjuster bolt in hand tight.

Then over a period of a few days adjust it more until the car does not rattle if left overnight.
So if it still rattles in the morning give it a few complete turns and wait for the next day.