Go to bakery supply place and get large cake pans (disposable)
spray well with pam or other release like vaseline (even better)
I made a dozen stones for my garden with SAKRETE in small round
disposable pans and they have made it through five winters...we
will see about this snowy one, though. I plan to make more this
summer. The larger size I have not done, I am just basing it on the
small ones, so do a try-out before investing too much $ or time.
Jane in Brooklyn