After reading through this thread:
...and working through R0J0hound 's neat system for 4-bit autotiling, I thought I'd have a pop at an 8-bit version and came up with this:
It uses a full 48 tile map, which means all those corners get properly filled. I've set it up so that at start it'll render any initial tiles laid out in the editor; it takes a few seconds to redraw the starting layout, but after that you can left click to add new tiles and right click to delete.
For this to work the bitwise values need to be mapped to the correct tile numbers - you can find the mappings in the tilekeys.json file
For those interested in delving deeper I found these two articles useful:
https://gamedevelopment.tutsplus.com/tu ... -cms-25673
EDIT: updated link to demo - improves initial tilemap generation (thanks to jobel for catching this) and fixes new tile creation on left click.