I till have an old Camp Trails Adjustable II external frame that weighs in a 3 lbs 14oz. I see these on ebay every so often and even purchased one for parts.
I also have a new Vargo Ti Arc titanium external frame that weighs just under 3 lbs.
Building my own external frames to adapt to packs is an off and on hobby of mine. I have progressed through different designs. Below is a photo of a day pack to which I added a homemade external frame. It has only two contact points and no mesh, all ventilation. I have since added a water bottle cage to it that I can access without removing the pack.
There are a number of manufactures of external frames. The 'traditional' styles that I see online are made by Kelty and Alps. There are a number of manufactures specializing in military and hunting packs that also make external frame packs.