The pictures below show how I made blocks for my stabilizer jacks. I used a 2" x 10" x 10 feet long board which was enough for all four jacks. I no longer carry cinder blocks. I'm to cheap to buy the orange commercial stuff. Its light and takes up very little room.
These blocks are designed to be used in the vertical position and not layed flat.
If used in soft soil, I do recommend putting a flat board under them.
I primarly use them for unlevel sites when the stabilizer's won't reach or barely reach the ground.