I have that issue, and it is a problem finding a snug fit.  I was not aware that Seek Outside had packs made for slimmer hips, so that might be your answer.  I do recommend trying on different packs at REI, since I've found some that do not claim smaller than 30" but in fact will snug-up adequately (Ospey Exos worked for me).  If you fall in love with a pack that does quit fit your hips, you can try a simple hack.  If you are good with a needle and thread, you can add a little foam to the hipbelt.  You'll need to encase the foam in fabric to keep it intact.  You might even have appropriate padding for this on packs, hydration belts, etc. that you no longer use.  Good luck!