Like Bert-o said get some plywood, 3/4" works well. When I used to crew at Sebring we'd bring four sheets of 3/4" plywood and lay down on the grass so we could work on the cars and it always worked very well.
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Don't bother giving me "rep", I won't return it and I think it is stupid.
Gracie Barra Clearwater
Gyms in Clearwater, St. Pete and Port Richey
pm me if you are interested in trying a free class at any of the gyms.