Excellent question, I just finished my own 4-pillar stands very similar to Skylans, except my posts are 3" round instead of square pvc and 26" high.
I'm filling mine with #9 Lead Shot on the very bottoms, (about 15-20%) then finishing with Silica Sand (to approx. 70% height of the posts) to prevent top heaviness.
I'm filling mine with #9 Lead Shot on the very bottoms, (about 15-20%) then finishing with Silica Sand (to approx. 70% height of the posts) to prevent top heaviness.