Well, for 8 feet long and , maybe, 4 feet wide, a single sheet
of OSB nailed to the top of the side forms would make a good
top of a mold....
This may be more trouble than it's worth, and I really go for
the idea presented of have a really stiff mix and just working it
till it starts to set.... HOWEVER.... if you pour cement into a mold
and wait till it firms up and then remove the top and trowel it, that
might work also....
I found the answers here very interesting as I have wondered
about the same problem.....
Up to now, I have just put down black plastic and about 6 inches
of gravel or base rock. It works really well, is cheap, and easy
Just my two cents worth,,
Andy in Eureka, Texas