Roy,
Like Gino said, all cutting and alterations should be done before assembly, if possible. I don't know your experience level, but in any case, extreme care should be used when working with hobby knives in this manner because it only takes a micro second to be in against the bone! OUCH!
Also, I don't have a "helo" spares box either. My box/es have everything from submarine to funny-car parts mixed in; Just carve & whittle & sand until the piece/s looks like what you want them to. Oh yeah, get a tube of Filler Putty also.
Take care, good luck,
Frank