What's wrong with a vertical slide? Does the keyboard end up being too small or something?

Looking on Amazon, I do see a lot of Bluetooth keyboard options, some of which are really teeny.

This one clips to your phone and slides out, but it looks like it's only for iPhones: https://www.amazon.com/Monoprice-Bluetooth-Pocket-Keyboard-iPhone/dp/B00KFSNOUA/ref=sr_1_3?ie=UTF8&qid=1468691406&sr=8-3&keywords=phone+keyboard

There are generic ones but you'd have to figure out some other way to attach it:
https://www.amazon.com/Bluetooth-Wireless-Keyboard-Computers-android-Windows/dp/B00LFHCUPA/ref=sr_1_8?ie=UTF8&qid=1468691406&sr=8-8&keywords=phone+keyboard
https://www.amazon.com/HDE-Bluetooth-Keyboard-Playstation-Smartphones/dp/B00H88HC4Y/ref=sr_1_5?ie=UTF8&qid=1468691406&sr=8-5&keywords=phone+keyboard

There's also some generic ones that clip to the phone but they're pretty huge and might be too clumsy to use standing up.