I did that for over 25 years , except that it was supposed to look exactly like the surrounding rock , this meant a lot of hand carve . Easy to do mechanically speaking , and in some cases we grafted onto the natural rock . When we were done , you couldnt tell the difference between the two , and couldnt tell it had not always been there . Best thing to do for a hide out , is get some land right up against a mountain , then excavate the living area , sort of like a split level home , then drill your steel directly into the surrounding rock . After the structure coat dries , finish out the inside work . Then shoot your outside finish coat in steps ( as you will need to hand carve and blend , you dont want the cement setting up too fast ) combination of latex press pad made from the surrounding rock , and hand carve detail textures . Let dry , paint , OH ! almost forgot , the cement should be integral colored to the natural rock ! , toss some plant litter on it to the same degree as the surrounding rock , if its slightly dirty , then you need to dust with dirt also . Your done ! You wouldnt know you were on a survival bunker if you were standing on it . Its all in the details , nature will do the other 50% . Anybody can do the structure , but you do need to have a person with strong sculptural skills to blend it correctly . I did it for 25 years , got sick of the traveling and instability of the industry , and changed careers . The cool thing is , a good many of the exhibits I did all over the country are still being seen by people every day . I know most of the tricks of the trade , making fake things look real . I have the know how , just not the money , or I would have done it for myself . Not just rock work either , trees , coral substraight , the whole nine yards , unfortunately , it does take a bit of money for the materials to do it right .