Hi Vikki. I think it would work, but I don't think you would be able to move it! That would be an awful lot of 'tufa, as well, probably cost about thirty bucks once you add it all up.
People here usually use an armature of some sort..styrofoam or chicken wire or whatever they can come up with, then add the tufa in layers over it. I've never tried anything that large, but there's lots of references here if you search.
(going to check out cottage garden :o) ) Wendy

Wendy, Thank you for your quick response to my question. Your warning about the size and weight of the "boulder" is well taken. I think I will start with the smaller rocks to place around my flower beds. Maybe later I'll try to do a boulder over chicken wire.

I really have enjoyed reading this forum and will come back often for some great inspiration.
Vikki