I've never got on that well at Montgomery - it's difficult to turn up there, checkout and simply rent. The one place that allowed this (some years ago) no longer seems to be around but the planes were in a horrific state of repair and when I pointed this out to the owner, he suggested I rent from somewhere else - which I duly did.
Outfits such as Plus One have a great selection of aircraft but seem to want you to go through a new member's induction course which I could never be bothered with.
I'd suggest trying Orion Aviation at Palomar - about 45 mins north of Montgomery. Very tidy, friendly and professional outfit with their own maintenance and a good selection of aircraft (one of the nicest C-172RGs I've ever flown). Palomar is a very nice airfield with an ILS. I'm in San Diego quite often and find it well worth the extra distance up to Palomar (even though I pass Montgomery on the way...)