Can you pre make these?

Sure, I did that. You'll end up with a lot of little pieces, not one big slab because the mold is the same thickness all the way around. If you do a lot of them you'll either have to find a way to keep them together or organize them or you'll have a great big jigsaw puzzle I had an old piece of formica and just loaded the mold up on it several times. I'm sure you could do the same over a piece of plastic. Make sure the concrete won't stick to whatever you put in on. I

Casting concrete with the QUIKRETE® Walkmaker® forms in a location other than where it will be ultimately placed is not recommended. The concrete when formed will conform to the surface over which it is being applied - typically this will be over an area from which sod / soil has been removed, and 2 to 4 inches of gravel has been placed and compacted to be relatively level and flat. If it is cast on a completely smooth surface (for example, an indoor floor with a plastic tarp over it), then moved to a surface which has some irregularities, the concrete may be subjected to stresses, and may be less durable. Additionally, the process of moving the 2 inch pre-cast may subject the concrete to additional stresses. Casting in the final location is recommended. Please contact the QUIKRETE® Technical Center directly at 1-800-282-5828 if you would like to discuss.