Sign is relatively small, use chemical anchors to tied into rock to use a ballast and pour aa concrete pad base with lots of rebar tied inot the anchors.
Frost should not be a problem if concrete pour directly on clean rock.
Engineer should be able to stamp this type of solution.
Everbrite is the main vendor. The franchisee typically buys direct from them and they ship to site for some one else to install. Though other companies may do Duncan signs, Everbrite is the only one I know that does the menu boards. We do Duncan in New York for several local franchisee's and buy the menu boards from Everbrite.