finasteride isn't for everyone, tis a fact JayMan.
Im determined to be a responder to finasteride (hence being on my fourth attempt to take it now) but I'm getting the signs of weird heart pain again, so Ill probably have to drop it. However, I'll probably just use dutasteride because I believe internals are so important, but that's just me
Anyway in advice for the original poster, diffuse thinning is hard to treat with topicals because you need to apply to a large area. It's easier if you have short hair, however.
I'd first like to urge you to consider finasteride, because it really is the best treatment there is for hair loss today. If you're dead set against it here's my advice for you.
Everyone's best bet against androgenic alopecia is a multi-angled treatment approach, inhibitting DHT and stimulating growth in as many ways as possible. For most people, they just take finasteride and use minoxidil and/or a copper peptide, but if you can't take finasteride things get more complicated (as I found out when i researched this after quitting finasteride a while ago) The following is my opinion of the successful topical-only regimen.
You will not only need to address your hair loss in a variety of different like everyone else, you will also need to
attack DHT in different ways, because topicals are less likely to be successful. Your options for this
spironolactone - quite cheap, even cheaper if you mix your own, proven as a topically effective anti-androgen in hirsutism, acne and one hair loss study. Works only where is is applied, no side effects have ever been reported when used topically. However one study found only the cream version worked, and this version might be hard to apply to your whole head. Needs to be used twice a day.
Fluridil - expensive, the only study was done by the makers of fluridil, and needs to be applied to a completely dry scalp otherwise it degrades. It's good for diffuse thinners however, because it's a liquid and you use 2ml of the stuff each day.
Flutamide - dangerous, only works through systematic absorbtion, stay away
RU - most powerful antiandrogen there is, no systematic absorbtion or side effects have ever been reported, and reduces androgens where it's applied to castration levels. The results from the studies on this are amazing, really. Now the downside - no one makes it as a product really

All you can do is import the raw chemical from China, at great expense, and mix your own. If you use the main recipe and use 2ml a day its about $100 a month, $50 if you use 1ml.
Revivogen - unproven, can be somewhat expensive, but uses Free Fatty Acids which is a different approach to antiandrogens like spironolactone or the others. Using free fatty acids against DHT has been proven. Also you get 2ml to use each day, so would be good for diffusers. Could be cheaper if you only need to use 1ml.
On top of using one or more of these things (unless you use RU i'd recommend using at least 2 to be safe) I'd
definately add a copper peptide to your regimen. Cheap, and easy to use, everyone should use them
I'd also look into
http://www.drproctor.com Prox-n I would recommend as it contains both copper peptides and NANO growth stimulants. If money is no issue I'd recommend proxiphen.
Good luck!