I think it is a case of getting the mix right, less hardener and more of the flexible compound and you should be fine, salix used some thing similar and added a colouring agent to make there toe guards back in the day they also used round the shoulders if I am not mistaken. I have found it works well. A little trial and error and you will get there. Only issue I have had when adding more hardening agent is that the glue cracks very easily and the glue begins to chip off. No other adverse effects have been noted.
5O's up started of very streaky, but hung in and got there in the end.