Well, making assumptions based on the one that fell over, your coffins appear to be rather on the shallow side, no more than maybe 6 or 8 inches deep. That might make it difficult to place weights in them. So I'll suggest this:
take a length of 1 inch diameter PVC pipe and attach pairs of them to the backs of the coffins, oriented vertically to the way you want them set up You want them to be two feet long if possible, otherwise as long as you can hide on the back of the coffin. Paint the pipe and pipe clamps to match the coffin. Next you'll need rebar of less than 1 inch diameter.
Then you'll take a sections of rebar as long as the PVC pipe plus 2 feet, and hammer them into the ground until the extra 2 feet have penetrate the ground. Use a hunk of 2x4 or 2x6 scrap at least 18 inches long and a rubber mallet to pound them in. You do this by placing the scrap wood on the end of the rebar and pounding on the wood. It will take longer but you'll refrain from damaging the end of the rebar (which would otherwise flare out and not fit into the pipe). This technique also provides the ability to use the pipes while attached to the coffin as guides to drive the rebar in perfect alignment for the pipes. By beating down on the scrap instead of the tip of the rebar, you can hammer away far enough away from the coffins that you won't damage them. This scrap and rubber mallet method is great for driving just about anything into the ground.
Another suggestion: Change your location to "Denver's NASA Wind Tunnel, Colorado."
