I might go for this solution:
Cinder block stand, filled with sand + AMC sylomer Pad 110 (25mm) (3 or 4) between stand and speaker.
Sub will be on the floor with 4 Pads.
You can check AMC Sylomer pad here:
sylomer pad | AMC Mecanocaucho
Main date here:
My question is: Do these pads work with such light weight (15kg in my case)?
Seems to good to be true.