i have 2 stimos that i have had for around 10 years was thinking of putting them in there for their retirement. id recomend building your base structure with chicken mesh than covering with a light cover of expanda foam, the foam once dry gives the wire strength and stability, after its dry and your happy with its form paint the whole thing with straight bondcrete this gives better adhesion to the morter mix, before the bondcrete dries you need to get your morter mix on there the ratio of sand to cement should be 3 clean sand to 1 cement this makes the mix a bit more sticky the more cement the stickier it will become, i also add straight bondcrete to the mix aswell say 3/4 of a cup to every kg of morter mix,than add water untill the mix becomes like a sticky clay but be carefull to much water and the mix will simply just slump of its structure u still want it dry enough to hold its form. if you do it properly you can get it to stick to the roof. when applying the morter mix make sure you put it on nice and thick no less than 2cm deepthis enables great carving once the morter is dry.now the part that i hate WAITING FOR IT TO DRY .this is very important start carving your nooks and crannys to early and it will fall if in large chunks so just be patiant and also dont leave it to long cause it just becomes to dry. the way i usually test it is rub your hand over it and if there is still moisture sticking to your hand its to wet but when you rub your hand over it and there is no moisture on it and some sand loose sand comes off it should be ok its kinda just one of those things that takes practice and time. when you feel it is dry enough take a butter knife small flat head screw driver and start carving away rembering that you only have around 2cm of depth to work with. try and print off a picture of a rock you like the look of and work off that when you have finished carving sweep over with a dustpan brush this smoothes over the cement and gives it a nice finish . some other things you can do is place some pebbles into it and get a bucket of wet sand and throught it quite hard at the morter and leave the whole thing to dry once the morter is complety dry the wet sand you threw at it should also be dry and than it can simply be brushed out leaving some nice little divits and creators behind. colour is up to you than seal with 2 coates of straight bondcrete
goodluck
