Cinder blocks?
I have a dual stand.... stacks 2 keyboards on top of each other.... it is rock solid. Wide base... and works OK for seated depending on the height of the seat, but the lower keyboard is easier to play seated.... but standing... works well on both upper and lower.
Our keyboard player used this rig standing....of course, she was a short girl so it was the perfect height for her.
The supports are in an X pattern and there is a brace with notches which allow the X to be adjusted> I currently have it in the furthest position..... which is the lowest setting. Moving the catch to the notches further in result int he keyboards being raised up by a considerable amount if you go all the way in.
The pic below shows it with 2 keyboards on it and unfortunately not much of the stand.
Two adjustable X's front and back.... welded to a bar on each bottom side... the bar is about 3 feet long.... the X's have a bolt through them to allow the X to move...and the rear (audience side) X has the bar with the notches and a clamp to secure the X angle at the required height.
With the big 3' bars on the floor, this thing is stable even at the higher settings. The red upper keyboard holder is removeable if only one keyboard is being used.
I have since retired the upper keyboard.... it's behind the door in my daughter's bedroom, and have fashioned a platform from wood to hold my laptop in the position above the Yamaha below it....