Until my Texblades arrive this is the best portable keyboard I know of: Stowaway.
I'm not endorsing that sale, just pointing at an example. You can Google for more info on them if desired.
Yah, they're old. So old the keycaps have Palm and PocketPC commands screened on them. I've used them with iPhones, iPads, Nexus tablets, Surface Pros, and Rasbperry Pis. They have excellent battery life, great key spacing for a touch typist, and fold up fairly small. I've tried several of the newer alternatives and for me nothing else hits the sweet spot of size vs key feel like these do.
Unless and until, of course, my 'Blades arrive.