If you want something that double-hinges away from the wall...
- Try www.ebay.co.uk if only for the various designs (£13-19-25)
- Pricey ergomounts are more for often-moved monitors

Coke cinder blocks crush easily due to big interstitial spaces.
- A monitor arm providing leverage is just enough to do it
- If the wall is bare, perhaps infill with a good cement mix

I recall you found how the black dust is destructive re staining :-)
Some were only "part cast" just after the war and are unusually thin.