There are lots of prototyping tools out there, though I find that a lot of them are focussed on designers. Their focus being that you have a Sketch mockup and want to turn it into something that looks and responds as much as possible like a real app. What I wanted was a prototyping tool that was aimed at the "before design" phase.
I then found POP. In short, POP lets you take photos of your hand drawn mockups, add tap areas and link them all together.
Here's an example of an app I'm currently working on:
To make drawing the sketches quicker and to give me a rough screensize guideline, I had a iPhone 7 sized stencil designed and 3D printed!
It's not 100% perfect yet but I think with a few more design tweaks I think it will be awesome. I'd love to have them in metal too.