On a tip from Jim Baumann, I replaced my buckwheat hulls (which I also liked alot, but heavy as noted) with BEER PONG BALLS.
No kidding. They weigh almost nothing and provide an adjustable yet very stable platform. Look for them in bulk on amazon or ebay, I think I used about 3x 144 packs for pretty full bag.
A few seem to dent each time when sitting but so far they recover (and make an unusual rainstick like sound as you leave the station, LOL) in use for about 1 full season.