There were a lot of engines running around back in the day with gunny sacks lain on top of the rockers.I personally put several of them in the $25 stovebolt sixes that I ran around in back in the early-mid 50s.I didn't know anyone made a product designed for that purpose.I'll bet the gunny sacks were cheaper.(They were free).We just had to make sure all the oats or feed was emptied out of them.