You've done a reasonably good job considering - well done. I agree with the diffuser - the sto-fen is not the best for macro diffusing. That said, it's good, but it's designed for people portraits, not bug shots. Put a VW in a F1 race and you won't get good results either. I'd recommend what macro junkie has done - it's LordV's (Brian) Coca cola diffuser. Yes, made from Coca Cola cans. Do a search on coca colar for the user LordV and you'll find instructions on it. Takes around an hour at most, works well.
Dave