I've tried stabilizing my SMBC with about 6 ounces of high-ratio shortening and even tried adding some meringue powder to a batch to stabilize it. I know this BC is not meant for hot weather, but I iced a cake and was doing some simple decorating and it was just melting and cracking all over. This was indoors in my kitchen that was not over 72 or 73 degrees. I really want to master this BC, as I think it tastes the best, but I just don't understand how you can decorate with it, piping, etc. and also how to use it as your icing and attach things like fondant cutouts or gumpaste flowers without everything sliding off.
It sounds like you might not be mixing it enough. To firm it up, try putting an ice wrap around your bowl. (they have refreezable ones for first aid-neck size) the coolness will help firm up the icing. Just don't over do it.