What might work best may vary due to type of soil and weather.
Grass might indicate some sod, I assume! That means it might get wet?
I find concrete pads break if they are even slightly on some higher spot.
For ease of getting them, if you have some big box store, etc. that will cut for you, I might lean toward wood like 2X8 cut to about 10-12 inches long.
If the ground is not too boggy to let the boards sink too far, the 2X is often plenty to keep from splitting and any extra sticking out too far at the ends, may just let the wood bow up, so no big help.
If you are in some of the boggie areas and the wood may sink to the point it becomes hard to recover, I might suggest double the wood. I tend to abandon it if I have to dig it up and deal with that dirt covered stuff!
An idea that you may want to consider if this is going to be a long term item to use over and over, I might offer an idea on placing them. When raining, etc. I found getting down on hands and knees to place the boards was a bummer.
We had a spare fork for use at campfires that had two points. I found drilling a couple holes in the boards and then bending the tines on the fork at a 90 degrees at the end and then something like a 40-45 degree up further toward the handle was a handy item.
Being 30-36 inches long and then the working end near horizontal let me move the boards all around until I got them directly under the jack pads as my wife worked the controls inside!
You may find that REALLY nice if dealing with sudden overnight snows!
Something like this fork bent to fit?
https://www.rei.com/product/884357/c...70001&store=32