I would go with the concrete idea, but you may be able to do it on the
cheap. Contact concrete suppliers in your area, and ask them if you can
have the extra cement mix after the trucks do their pour. There is
almost always a surplus of cement in the mix truck after a
foundation/footing pour, and they have a limited amount of time to get
it out, and it can be a problem of where to put it (it is often buried
at the jobsite, to the chagrin of a homeowner who later tries to dig
out the spot). If your location is not too remote, they will gladly
take it to you. Just be sure you are there when they come and have the
job prepared, otherwise they will dump it in a pile on your property,
rather than have it set up in the truck mixer. It will take awhile, be
patient. They will also want to rinse the mixer with a hose, no problem
for this type of pour since concrete quality is not that important in
your application. Best wishes for your project.-Jitney