Have a dozen of various buoys in my garage. That's a result of kiteracing in 2009-2015.
Most of them were custom made in Poland by WATS.com.pl
this is a result of our testing - training buoys:
The weight was 4kg ish for a 20cm diameter and 150cm long pillar buoy. Price 60€ each.
The valve - the same as for SUP boards.
Additionally my buoys had straps to carry them on my back.
However the best for kiteracers according to our national team were buoys made of a kite strut bladder
The lightest ones and the easiest to carry.
Take a biggest strut bladder and put in an red, yellow or orange canopy. Attach 2kg weight to it which you carry by connecting it to the harness hook.
You might add a large flag to a top of the bladder too.