I really would not use a cinder block... I like solid blocks of wood.
I use a couple of 4x8 blocks that are only around 18" long and stack them.
I can break a cinder block with a small hammer... Or just dropping it..
The cinder block is heavier than my chunks of scrape wood, so there you go..
๐Good luck!
Mitch
2013 F150 XLT 4x4 SuperCab Max Tow Egoboost 3.73 gears #7700 GVWR #1920 payload. 2019 Rockwood Mini Lite 2511S.