This is for a leg holster, but same can be used on a sling. I need to extended the quick release band by 6 inches.  I need an extension buckle something heavy duty & not junk. Something like this....
thanks

Looks like the above picture you posted is actually a helmet chin strap extender...

If you have an open ended sling one of the Viking Tactics Quick Release Extensions would probably work great:


https://www.vikingtactics.com/product-p/vtac-qre.htm

Also, the Blue Force Gear Quick Release Kit (8" webbing):


https://www.blueforcegear.com/quick-release

If you don't have an open ended sling you need to find out what size/ type of buckle it is. ITW Nexus buckles are usually whats used for sling equipment buckles: https://na.itwnexus.com/products/categories/military-defense-buckles