I added a gas spring from the boom to the stick. I only found the correct length, stroke and force on Aliexpress:
 - 6kg (60N) gas spring: 
http://www.aliexpress.com/snapshot/7601366858.html
The sleeves and spacers are cut from 8mm and 12mm stainless steel tubes. The pin is also gotten from Ebay:
 - 8mm (ID 4mm) tube cut to 13mm length: 
http://www.ebay.com/itm/121600172085
 - 12mm (ID 8mm) tube cut to 4mm legnth: 
http://www.ebay.com/itm/121762512929
 - 8x30mm pin: 
http://www.ebay.co.uk/itm/121851516316