I'm correcting this -- I was obviously FAR too tired when I wrote this last night. Struvite stones ARE the ones triggered mostly by too alkaline ph in the urine (needs acid) and they are the easier of the two to get rid of. My original post here was absolutely wrong.
Cranberry helps with urinary tract *infections* (which an infection is NOT "stones" -- two completely different things) -- because cranberry is largely glucose. Glucose will bond with infection cells and help them pass thru the body and NOT stick to the urinary tract walls.
Cranberry is slightly acidic. Not hugely so but it would be a MILD help for the a dog with struvites.
The better alternative, if you are dealing with infection (NOT just stones -- if stones are the *only* problem that's a whole other thing) -- but try D-Mannose. It is magnesium in the form of a sugar -- yes, another kind of glucose.
It's a supplement -- my favorite brand is NOW Brand Foods -- but you can find it all over the place online. It's not cheap -- but it really helps with recurring infections and it's not acidic.
However -- I would still caution you in a huge way **not** to do this alone. Why not contact someone like Monica Segal (http://www.doggiedietician.com ) and get the right diet for your dog. Sabine Contreras (Mordanna -- and I know folks on here have her url) is also very good. They will write a balanced diet that takes such medical issues as struvites into account and I believe both of them will work with a raw diet if that's your preference. I know Monica pretty well and I know she has the medical background to work with struvite stones and help you plan an optimum diet that will work AND be balanced. (I'm not recommending Monica over Sabine .. I just know Monica better is all -- both are highly qualified and very good.)
I don't blame you for wanting to do better food-wise than Hills. But struvites is a pretty severe health issue and you really must make sure you do something balanced or you'll worsen it rather than making it better.
You can easily acidify the diet by even adding some tomato juice to the food, or pineapple or such. You can also give a bit of Vit C (ester C is normally pretty well tolerated but start LOW and increase gradually to make sure you don't cause diarreah).
My other suggestion would be to pursue something like TCVM (traditional Chinese veterinary medicine) -- there are things that can be done with acupuncture and Chinese herbals to help reduce the tendency to develope those struvite crystals. I have one dog myself, who is already prone to struvites and she's just a year old.