I've never tried this, but this is something that may work. You could build a mound of earth about the size and shape of your desired rock. cover it with thin plastic like a garbage bag. Lay on a layer, perhaps 3/4 inch thick of hypertufa mix (use strenghtening fibers in mix). Next cover the area with hardware cloth, forcing it into the mix. Then add more mix forced into the first layer and hardware cloth to give about 1 1/2 inch total thickness.
The outside can be contoured to suit. You might even make up some mix with tan coloring dye and put it on as you build up the top layer to give the appearance of different color strata. You could also scratch grooves in the surface to also resemble strata or irregularities---to give it character. When finished, you can wire brush it and/or mud it over with a soupy mix until you like the way it looks. Perhaps others can add to what I have suggested.

Here is an album of hollow rock, faux rock making.
Your rock will only be as strong as your armature. I think they can be made to look real. They are not super light weight but a 2-3 man rock made with a hollow armature becomes a 1 man/woman rock.
You can cover the wooden box with plastic, then cover the plastic with a wire armature and proceed with your mix to cover. A hollow rock can be made to look like a rock with stains and diluted paints, and the way you add texture to your surface area.
This album I put togetehr of rocks I made and also of a class I gave this last spring.
I recently placed three large rocks in a flower bed and unless I say they are fake I do not think anyone knows.
And it is a lot of fun,
Dena

{{gwi:75063}}
by dena6355

Here is a link that might be useful: making rocks and hollow concrete structure

Thank you both! Those are great ideas. Dena, your rock looks very realistic. Hopefully I can pull off something along those lines. Fortunately, these clients are very easygoing, so I doubt they'll be too upset if it goes wrong. I just don't want it to look like one of those horrible fake rock faces you see along freeway retaining walls.

Dena: Wonderful rocks! I'm in need of much larger rocks than either my DH or I can tote & have been thinking of trying to do just this.

A few questions:
Do you finish off the bottom? If so, how? After the rest has cured & you can turn it over?
Any special methods of adding color? Watered down acrylics or crete dyes?
The burlap method-- saturate the fabric & manipulate that over the form? Since you are covering w/ more tufa-- what is the prupose of the fabric? Does it help w/ finer shapes?

Thanks for all the pics!

We live in a community with an HOA whose snooty-nosed forefathers had all house lots landscaped with an irrigation backflow valve in the front yard, covered by an oh-so-obviously fake rock. I've yet to confirm that such cover is required by the covenants, but expect they would impose a ruling as such.

Are there any other options out there besided the "fake rock"? I also don't know if the valve itself is of a metal which needs to be protected from the weather, though it would seem to need some protection to avoid dirt build-up, etc.

Here is a link that might be useful: {{gwi:75062}}