i was thinking about a rock as well meremily...with a chair base. also wanted to take off an arm so it would look more loungy.
i did find this for you:
http://www.wikihow.com/Make-Fake-Rocks-for-Your-Pond
the person that suggested this site wrote this about it:
The way I have found that works best, is to take something the general size of what you are looking for (we made a fountain with a rock the size of a chair-ish next to it) and cover it in wire (you may want to wrap the object in a sheet so it doesn't get all dirty and can be reused when you are done). Then cover the chair, or whatever, in expanding foam (you can get it from a hardware store). It is strong enough to sit or stand on, but looks rock-like (textured). It may take you a day or so to do the whole project, due to drying time of the paint and the foam and such, but it's a fairly low work load to get it done...and its really entertainig to watch it expand! good luck!
a lightbulb just went off over my head-and i think i'm gonna do this: (less expense ??? i believe, easier to transport, & more comfy)
get a saucer chair and recover in pearlesant fabric, cover metal chair bits in whatever - plants, paint to look like coral/rock. think of sitting inside a pearl

i can always add a tall thin fake rock or other for backdrop.
wallyworld has 2 sizes.
$19 for smaller indoor chair.
http://www.walmart.com/ip/Mainstays-...ndingMethod=rr
and $39 for larger outdoor chair
http://www.walmart.com/ip/Mainstays-...id=N/A&veh=cse
just brainstorming----
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