....one simple and cheap suggestion, if you opt to infill the hollows in the blocks with sand....
Get a box of those heavy-duty trash bags and put one in each void to hold the sand.
One brilliant day pending, you may want (or have to) dismantle your 'stack'....and +/- 2.5 cu.ft. of loose sand will really totally trash not only your 'tude, but likely the floor....
It ought to make for one stable platform, though...'bout 500 lbs. worth, on just under 2 sq. ft. ....
Hope the floor's up to it...🤞